Flak for Engineer and Nano for Medic are, in my opinion, must-have and no brainers. For the other classes it's situational. Indoor fight? Nano-weave may prove more useful. At an amp station? Flak armor will help more if there's tanks outside. Personally I use flak on most of my characters save for medic as I like to play recklessly and don't want to worry about prox mines or grenades.
Flak and Nano both have their merits, but Nano is the better overall choice since it's a straight health boost. Flak only mitigates splash damage, which is one out of the two most common ways to die (the other one obviously being bullets). Nano helps deal with both damage sources. Nano 1 is one of the cheapest ways to increase general survivability.
http://tvtropes.org/pmwiki/pmwiki.php/Main/JackOfAllTrades aka nanoweave vs http://tvtropes.org/pmwiki/pmwiki.php/Main/CripplingOverspecialization aka flak armor. I guess it's situational. On one side you have a generic boost to hp, that gives you an extra shot and maybe lets you survive a headshot if you're at full health and shields. On the other hand with flak you take half damage from explosions so while holding in your hands an exploding grenade, taking a RL or tank shell to the face may not be a good idea it lets you survive "close" encounters with them. - HA: nanoweave, with resist shield its absolutely the best choice for combat. - Medic: being able to take that extra damage and heal myself is nice. - Infiltrator: always a good thing to survive an enemy infiltrator HS if he's far enough. This is also the class with the least HP so I kinda like that extra defence. - Engineer: lvl1 nano, only use this when I spawn an MBT or ESF so wouldn't I guess it goes to preferences. - LA:Flak armor for the extra explosion resistance when you need to detonate your C4 too close. - MAX: flag armor. this should almost be a default for MAXes since you're going to get a lot of explosions aimed at you, nades mines c4.. you name it. It's better to have 50% more flak armor(with flak lvl 5) then the 7.5% more resistance you get to small fire arms with kinetik armor at max lvl(from 80% to 87.5%). This ofc is my choice of gearing up and oppinion, others might disagree and/or have better options. In then end it only depends on your play style.
