At it's most fundamental (yeah, that's right - straight into the meat of the article, no exposition, nothing! lol) there are two ways to win at this game. By killing, or by surviving.
Obviously, the game is slightly more complex than that, the two aren't (of course) mutually exclusive, and both require a little bit of exploration and explanation before we can proceed...but, before THAT, let's come back to that 'winning' comment.
This article series is designed, primarily, to help those starting the game, coming back to the game after a long absence, or those starting to move into the sphere of not going 'pewpew' in the basement. In no way are they exclusively for Tournament Gamers, or even just for novices, experienced gamers will hopefully be able to gain from them too, especially if coming to a new army, or new system (ie, [re]forme[d]r Square-basers.)
Aiming to 'win' is a basic fundamental of the game. While certain aspects, such as Capture and Control as a mission type, Jervis' Standard Bearer articles, and the general attitudes of GW staff all poo-poo the idea of playing this game with any vague sense of competition, this is a game about WAR. We use TOY SOLDIERS, and, again, it is a GAME. Games are played to be won...we play them SPECIFICALLY, over other games, because they are fun, but if we couldn't win, I can't imagine that being fun for long, amirite?
This article, perhaps more than the others in the series, deals with this fundamental because it will attempt to help you determine what way in which you intend to go for victory. For players of Sid Meier's Civilization games, this is akin to choosing what Civ and Leader you will be before starting Civ IV - if you are Augustus, a Cultural Victory or Space Race is unlikely to be your first option, any more than simple Domination the domain of Rameses.
If you're wondering why 'winning' is so important to the question of picking an army, it is because it determines everything about strategy and tactic from here on in...you wouldn't play Tau if killing to win wasn't your thing, any more than you would if that was your thing with Chaos.
If winning isn't really a concern, then you don't NEED any advice, and, frankly, should never give any either.
It's like this. If you don't care about what's objectively good, then you are into a subjective measure of comparison. If this is the case, you can, essentially, be called choosing your army on a measure of 'cool'. Perhaps you like the fluff of Legion of the Damned, and therefore use an entire army of the models, including 3 max sized Elite units? Great, whatever keeps GW afloat for the rest of us. Maybe you use 20-strong Blood Claw units (they go to 20, right? Not into checking...detail irrelevant) and run them, or WHATEVER. You use what YOU choose to use, and I cannot possibly ever tell you what you perceive as 'cool'. I can perhaps influence you, and can certainly tell you what I think is 'cool', but it need not affect you one iota.
If this is your basis for unit choice, good luck, enjoy yourself...but PLEASE do not try to impose your criteria on anyone else, as we have our own subjective biases, foibles and preferences, and no more wish to be told to think something is cool than you do. People who wish to write so-called 'Fluff Armies' do NOT want advice, they want validation from faceless internet people they fervently wish to see as peers - this is not wise, or healthy. If YOU think your army is fluffy, cool, or whatever, that should be enough for you - but if you want it to be GOOD, then the internet is a good place to start learning.*
Now that's dealt with...
As said in the opening paragraph, there are two ways you can intend to win games of 40k.
The first, and most obvious of these ways is to cripple or destroy your opponent. This is the kind of thing at which Tau, Blood Angels, Space Wolves and Guard excel at, and Orks, Daemons, WH and CSM aim to do, to varying degrees. This is far from a definitive list - these are examples, and it is entirely possible to craft lists that attempt NOT to win in this fashion...at least from the 5e Codexes. Tau, WH and Orks, in particular, cannot play any other way. CSM and Daemons though, are debatable...in fact, whether or not they belong on this list at ALL is debatable.
Naturally, tabling the opponent isn't truly a requirement of winning in this style, though it obviously helps.
What *is* important is crippling your opponent's capacity to endanger your victory, and the 'ethos of continuation', as I choose to term it - the practice of dedicating turns to maximised firepower, over manoeuvring.
Firing and moving are not necessarily mutually exclusive, of course, but it is clear to anyone with a basic grounding in the rules that an army's critical firepower radically increases when stationary. For now, we can largely ignore the fact that Daemons and Orks achieve most of their kills in combat, rather than at range - as their ranged elements are so vital to their success. That is the inherent weakness of the principles upon which their Codii are predicated, but THAT is a topic for another time.
So, 'ethos of continuation' - what this essentially translates to is something like a RazorWolf list, an army that, despite incurring losses, still puts out a prodigious amount of firepower, hoping to smother the foe under a barrage of Blasts, Templates and failed saves. Having a single turn of overwhelming firepower (cf. Alpha Strike) is FAR less important than having the tactical stamina and resilience to outshoot the opponent for 5 turns plus. In terms of Orks/Daemons, this is slightly different, as I said. In the most basic terms, you need to rely on the actual 'shooting' for as little time as possible, and use it as a means to get into a position where the more treacherous business of CC begins. If you can keep your dangerous elements fighting for the whole game - and maximise your dangerous elements BEFORE reaching the table, then you have a foundation to win in this fashion.
When an army like this faces off against an unfocused army...it can be quite a magnificent sight. Unrewarding for both players, sure...but who doesn't like to see the trash get taken out? lol
Now, the other, opposing ethos I wish to mention is one of survivalism - the 'ethos of perseverance' if you please. This is the base design concept of the Eldar, and also applies to Tyranids**, Space Marines, hopefully to the rebooted Necrons (at present, the Necrons are FORCED to try and play this way despite being built in the opposite paradigm above. Screwed up, huh?) - Blood Angels, some lists anyway, are exceptional examples of this ideal - survival at the cost of destruction.
[Any experienced player reading this probably recognises elements of both in most lists, and is wondering why I'm being quite so simplistic...in which case you're missing the point. If this article helps you in no way, then you either don't want the help, or don't need the help, in which case it isn't for you, and I'd much rather have your feedback on it as a guide and teaching aid. ;-) ]
The 'ethos of perseverance' dictates that we choose units that will be hard or even difficult for the opponent to destroy and therefore remove the threat value of. A good example (just about) is an Eldar Wave Serpent. Overcosted though it is, and incredibly fragile as the guys inside are, this is one of the most survivable units in the game, partly because it gets cheaper Extra Armour than most current Codexes. At the same time, the humble Wave Serpent is hardly blessed with a sterling armament - spitting out up to 7 S6 shots, or 3 and a S8. Wooo. Frankly, it's more disappointing than that time Spider-man got 'reset' and 40 years of history were wiped out with the stroke of a pen. I wanted to see him try to live with his identity now known. Civil War fallout could have been the new Sinister Six. Sigh.
Er...annnnyway...armies based on this ethos are more concerened with getting their Troops where they need to be, and keeping them there long enough to get the job done, while getting their other units in place to prevent the foe doing the same. Horrendous losses are perfectly acceptable and almost to be expected - all that matters is the endgame. If this sounds wrong to you, please, think. This is Warhammer 40,000 5th Edition: Tactical Espionage Action. Killing is optional. Being James Bond doesn't mean killing - look at Roger Moore - and neither does wargaming any more. Of course, the things that are threats to your plans still should be killed where possible...but don't get sucked in. Always focus on three things:
1) The Mission.
2) The Mission.
3) The Mission.
Got it? Good.
Essentially, you want to pick an army that best fits whichever one of these camps you see yourself as wanting to be a part of...and you go from there.
An important note: - while 'Buying the Codex' is ALWAYS step one for starting a new army (if you don't like the rules, the feel or the fluff, you probably won't want to play them afterall...) part of being a decent competitive player is buying new Codexes anyway. You need to stay at least on the same level as the competition, and to do that you need to know them. What I'm saying is - If you want to be in the top half of players or better, you need to own every 5e Codex, or at least have regular access to them. Ideally, you would own them all too...but, let's be honest, GW don't much help us out there.
Here endeth the lesson...
* - As good a place as the internet is to start an education in the ways of the Competitors, like with so much else, the only real way to learn is to do. No more than you can learn to drive by watching Formula One, you can't learn to overcome similarly skilled players (nevermind better ones) by reading here, YTTH, 3++*** or anywhere else, without regular application of the things you pick up - and more than a few pratfalls along the way.
** - One of the main reasons Nids are Epic Fail in the hands of a lot of players, to my mind, and certainly a lot of Internet Loudmouths (looking at YOU, Warseer!) is because they can't/don't grasp this simple concept. The Tyranid army is NOT designed to kill it's way to victory like it was in 4e. It's designed to outlast the opponent, stopping them from killing it's vital elements by crippling them with their 'inferior' shooting, and dealing a crippling blow up close if the circumstances permit - or simply squatting on Objectives and layin' eggs.
*** - If you think I'm trying to list EVERY useful Blog out there, you got another think comin' homes. I name the Shark Tank, because it's the biggest, and original. I name the Aussie, because it'll salve his wounded pride when England somehow steal the Ashes. I name me because...well, duh.