Quite certain about best build now

[Wildeyn]

Assuming that you are planning to take your character into hardcore and beyond, I am now quite certain that the best build is:

Infiltrator, with assault rifles instead of the widow special sniper, and probably with Improved Geth Shield Boost (75% shields, 10% weapon dam for 60 seconds, and the great thing is that it is a power that instantly resets your shield in addition to boosting them). Main weapon is viper sniper rifle, assault battle rifle is used at times to conserve sniper ammo.

Then, you rarely use tactical cloak. Mostly, you just use the 'free sniper adrenaline rush' (time slowdown) that comes with your class passive ability combined with re-activating your shield with Geth shield boost. I am comfortable with not constantly using my powers; I instead tend to like to have my powers un-cooling and ready to use so that I don't get caught with my pants... or shields... down.

Viper rifle is preferable to the Widow because it lets you split up your shots and damage much more appropriately. It is more ammo-efficient (err... heat sink efficient) and allows you to take better advantage of the free sniper zoom slow down without having to reload constantly. And a full unloading of all 12 rounds, such as against a scion, does more dam than a widow shot.

Tactical cloak is really cool and powerful, but it also opens you up to those freak occurrences that get you killed. I'm no veteran of shooters (I'm more of a strategy guy), and I am nearly done with hardcore and I haven't died at all. I was totally stupid once on veteran, for my first playthrough with this char, and I died on Horizon totally unnecessarily, but that was my only death on veteran difficulty. I mostly just use tactical cloak against heavy mechs and sometimes against those Praetorian ships.

I use heavy incinerate when it is safe to save on ammo, and I have 1 pt into AI hacking. I have squad disruptor ammo that I use once in a while. I don't bother with cryo ammo.

My squad against non-synthetics has Jacob and then Jack once Jack is loyal, paired with Miranda and then Thane. That way, I have either squad incendiary or squad warp ammo, coupled with the ever-pleasing pull/warp combo. Against synthetics I supply the ammo and generally take people with overload when I can. The new download of the eviscerator shotgun helps Jacob and Jack be more usable.

Sentinels with assault armor are obviously quite great, as are adepts with singularity, but once you hit hardcore difficulty, and everybody has an extra layer of armor or shields, I think that the infiltrator is noticeably better.

[Aqua-chan]

Infiltrator is my favorite class, but I attempted to bring my Infiltrator as well as a Sentinel through Insanity mode. Actually, I started them on Insanity+, having re-imported both characters after playing ME2 with each once already. Even with all the best skills capped out from the get-go, Infiltrator just didn't have the survivability the Sentinel did. On Hardcore and lower difficulties they're great, sure, but I feel Infiltrators alone don't have the necessary skills to dismantle the plethora of enemy defenses you see on Insanity.

It's doable, of course. Just not nearly as easy.

Like I said, Infiltrator is my class of choice, and I was able to get into Sentinel pretty easily. Both are back-line combatants, so adapting to strategy is simple enough.

Sentinel Build
Throw - 3pts to unlock Warp
Warp - 10pts (Heavy Warp)
Tech Armor - 10pts (Power Armor)
Overload - 6pts
Cryo Blast - Ignore it. Useless.
Defender - 10pts (Guardian)