I used to use Flak (only lv3 though), but recently changed to Nanoweave, which has actually saved me a few times leaving my on a slither of health were I can potion up and wait for shields to recharge. The extra bullet you take can be the difference between winning a shootout. I think Flak needs to be certed higher to get the most out of it, but Nanoweave 1 for 1 cert gets you 1 extra bullet worth of defence so depends on how much you have to spend. Apparently Nanoweave 5 gets you another bullet's worth of survival against the faster firing, low damage per shot weapons but was quite a lot of certs IIRC.
I do nanoweave in close combat, flak if I know a vehicle rush is coming or if there's a lot of air, and shield capacitor if I'm doing long range support (since I usually don't take enough damage to worry about health at those ranges, and a faster shield recharge means I'm out and firing more). Generally, you'll want to use grenade bandolier or munitions pouch if you find yourself saying "Man, if only I had some extra grenades/rockets!" Those two are easily the most situational, but just as good in the right circumstance as the other three.
For general purpose use, probably flak. There's so much explosion spam in this game, it's pretty much the only defense available. There are situations where you'd want the others, however you'd have to ask yourself if those situations are worth 1000+ certs for an extremely minor advantage.
Depends on playstyle really. I've never found flak useful at all unless there are tanks. Avoiding grenades just isn't that hard since they can't be cooked. I use nanoweave since it works REALLY well with Resist shield. The ammo/ rocket/grenade pouches could be useful, but I run with an outfit, so there is always an abundance of ammo lying around. The shield recharge is kinda pointless in my opinion. It regenerates just fine as is.
125 health at rank 5, with resist shield up I guess it's 168.75? Not that well. It worked a lot better when nanoweave was just damage reduction, since they seemed to stack.
Currently using Nano1 because I was dumping certs elsewhere for a while and didn't think about picking a path. Since I have my Adrenaline 5, I honestly feel like my survivability isn't going to go up much in infantry combat, so I'll go Flak or Pouch. Flak seems decent against nades, but if you meet a lolProwler you're still boned without him having to reload unless he's a bad shot.
Got Nano 5 and Resist 5 and don't regret it at all. Gotta agree with our beloved leader Levelcap it's really good. You will probably use more resto kits, but you will never be out of shield.
Nanoweave 2-5 don't matter as much in CQC, but beyond that 10-15m range bullets do variable damage. I've survived with a sliver of HP countless times at nano 4 and 5. Also note that nanoweave 3 allows you to survive a headshot (or 2 body shots) by the 800 damage bolt rifles at 115m, nanoweave 4 reduces that range to somewhere around 95m, and 5/5 is about 75m. Gives me piece of mind I won't get instagibbed (as often) when standing still with a 6x scope for decent bloom.
Munitions Pouch is the only cert I run with in the Suit slot. I also have maxed Shield Capacitor, which I love, but I just don't use it much. My philosophy as Heavy Assault is that it's my job to pour as much ordinance down range as possible, and then die a glorious kamikaze death or try to go on a stabbing spree after I run out of ammo. In a game where death matters so little, you might as well focus on doing as much damage as possible in the shortest amount of time.
