I've played all classes but engineer, with differing success rate mind you. However, there's a few things to say about the different armors. Mainly nanoweave actually.
Max nanoweave sets you back 1211 certs. For this you gain 25% more health. Sounds good right? Well, everyone has 500 health, shields are another 500 (400 for infils). But nanoweave, as far as we all know, only covers health. So that's 125 extra hp. To put that in context, that's 1 body shot from a TR SMG at 10m. It's 0.6 of a bullet from a Gauss Saw at 10m. Generally it is regarded as 1-2 bullets extra under ideal circumstances. In other words, you are not likely to notice its existence in 99 out of 100 cases. In my opinion it is a complete waste of certs.
However, it does have a few redeeming features. Nanoweave 4, from what I've heard, will let you survive a mine with minimal health. Nanoweave 5 means you will survive a sniper headshot with minimum health. If the Heavy Assault resist shield was 100% reliable and not randomly bugging out it would mean that you gained extra health as the resist shield resists 45% damage. However, the resist shield is bugged right now and doesn't always work.
I use nanoweave 1 since it's 10% for 1 cert when I start a character out. However, I switch it out as soon as I've bought some other stuff.
For the light assault I run grenade bandolier, but as Sharpe pointed out, you can become resource starved very quickly. If I'm not running grenade bandolier I run flak armor.
For the medic there is only one option and that's flak armor. You will be exposed to so many explosions!
For the HA there is only one option and that's flak armor. I know some people disagree with me as they run resist shield or what not. However, I'm pretty sure they can't actually back up nanoweave being worthwhile with actual numbers. When they 'feel' they survive better I think it's a placebo effect in 99% of the cases. Of course, when resist shield works it's nice. But it's unreliable and doesn't protect against everything. You'll be at the front lines, and the front lines ALWAYS EXPLODES. Flak armor, get it.
For the engineer I don't know for sure but I imagine it would be grenade bandolier or flak armor.
For infiltrator it's grenade bandolier or flak armor if you're running SMG. If you're running sniper I'd suggest flak armor or nanoweave. Flak armor will help against airplanes and tanks while nanoweave will help you getting counter-sniped.
So why flak armor? It let's you run over a mine taking minimal damage. It allows you to survive a grenade, grenade launcher, rockets, tank rounds, libs, esf's and anything else that goes boom. I'm not entirely sure about this but I THINK it only works when it's not a direct hit. So that's a bit of a downside. But on the plus side, most people are shit at aiming so they rarely hit you straight on with their explosives.