Well, the space marine armour has to be there. Whatever chapter it is, it always looks badass. If space marines are meant to be such massive badasses capable of withstanding far more than the already badass-armoured imperial guard, they've got to look like it, and they've also got to look like they can liquify a face with a punch. The massive armour does all that while still looking mobile. I'm not such a big fan of chaos armour, as...well, could they be trying any harder to look evil? Seriously, all the spikes and chains and what appears to be skin, it's a little over the top, but then again, what about WH40K could be described as not OTT?
All of the armour sets from Earth defense force insect armageddon deserve a mention. The soldier armour strips away the BS for a simple, starship trooper-esque design, the engineer armour looks like a support class should, like he's linked in with god knows what under there, the jet armour is just goddamn awesome, and the heavy armour looks like a splicing incident between a man and a tank.