First and above all. Get the ones that look cool. Really. Make you you love how they look. They are going to be your guys, you're going to spend a whole lot of time with them. So say you hate the whole Biogentic thing of the 'Nids. Yeah they are kinda idiot proof of rushing the enemy, but if they aren't fun, then why waste money on it.
I suggest Grey Knights
If you think that's awesome then that's the first step. Getting the guys and startagy will come later. Get the army YOU think is cool, not because someone told you or you hear people say For the Emperor a lot
But if you wanna short rundown here you go (I put my suggest for you at the bottom)
Space Marine: Good at not sucking. They don't do anything really right other then exploiting an enemy weakness. Like Melee against Tau or shooting against Nids. Smaller Numbers means less guys you can field to hold objectives and get hit
+++++ Dark Angles More elite Space Marine, better shooters in some respects, and a unique set up to army construction
+++++ Black Templars Men in Power Armor ready to beat the everliving daylights out of you. Really fun, but you give up long range fighting
+++++ Space Wolves/ Blood angles: Sorry, can't help ya here with the new rules
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Chaos Space Marines: Again good at not Sucking AND get funky powers. But as such, they come in four groups. This is a tough one, and I'm sure someone can give a better summery. Think of them as Space Marines more tooled up for a fight, and have access to dark ones that can trick out their abilties in combat. But at a cost. Not the most dependable, all the time.
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Eldar: Quick and fast. Ninjas of 40k. And yes, ninjas. You catch them off guard, that tissue armor ain't holding up. A good share of guys, and good fire support and CQB fighters. But if you get the wrong guys againt the wrong target. You're done
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Dark Eldar: Much like above but very outdated and much scarier in close combat
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Guard: tanks tanks and big heavy tanks. Oh yeah and a gajillon guys with angry flashlights. The main force can get beaten up pretty bad against a skilled opponent, but the tanks will shell him into next week and those guys can hold their own with orders
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Orks: Tough'n scary and not to bright. The tanks are pretty shotty but do have some power behind them. And that's the thing. You can trust them to hit something hard, not shoot at it. Lots of guys to fight with and can have a WAAAAAGGGHHHH which can be a game changer
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Tyrnids: A gajillon and a half guys. Even a few with "Without number" the base guy is a pushover, but add like 50 of his friends. 20 of his cousins with razor sharp claws, their big scary carfix Uncle that can take a heavy shelling and keep ticking and you got a mess load of trouble
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Tau: PEW PEW PEW PEW PEW is the way to go. If you like shoving gun fire down their necks, this is the way to go. Very very hard hitting guns. Catch them in Melee and they slump over
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Deamon Hunters: You'll going to field MUCH less guys then even a space marine player. But each one of those guys in armed to the teeth, and is a fantastic fighter. Done right, a grey knight squad can wreck whole army plans. AND if you want they have rules to be added into other forces (like Space Marines and Guard)
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Sisters of Battle (nuns with guns that have all the fun) Okay, think of space marines. Except more of them, more flame throwers, and the ability to use a miracle. Yeah, pretty awesome. And they are all chicks so that's a plus =D But really, one of the only things is that I notice they don't do well defensively. They need to hit hard and fast to do damage. Also they are a bit out dated but can be added into other armies
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Necrons: Okay so people call them slow. I like to think undead zombie with a lot of FU potential. First off, they got the same save as a Space Marine. Which is rock hard. Next they have the abilty to come back after being downed. and there are ways to make that re-roll making it VERY hard to kill them. Next they have powerful weapons and the lord can do some crazy stuff. The downside is, they are look the same. You play one Necron army, you kinda play them all. Also if they get their arses kicked in Melee, wave them good bye, they are toast. Also you need to use them right as the enemy needs to only kill 75% of the guys before you are forced to phase out and lose the game. But I play a guy often and I never get him to phase out.
And who do I pick for you? Hmm.... Eldar. Yeah Eldar. They got a good deal of range support (snipers, anti tank teams, to tanks that will make your opponent cry) and the close range guys (S. scorpion H. Banshees or the Harlequins) will make a mockery out of even the toughest units
