Assault rifles: Harrier, Typhoon. Everything else seems underwhelming. Yeah there are some good other ones to use but really are there any that are actually better and more worth while to take with you than those two? Unless weight is an issue but in that case you are probably using your powers more anyways and the weapon is secondary to that. Maybe the Lancer gets an honourable mention here but after using it, I was totally underwhelmed. Don't even get me started on the Spitfire, what a useless POS that is.
The Harrier is definitely my all-time favorite gun in the game, but generally I stick to the Mattock in MP. I don't really care for the Typhoon, but favoring power-based classes kind of takes the appeal out of it. In single player, it's always a companion weapon for my tank Vega.
Pistols: For me there is just no reason to bring anything else with me but an Acolyte. It's got to be the best sidearm in the game in terms of quickly dismantling shields to then use a power (or another weapon that is good against armour). I just don't see the reason to bring any other pistol except for the rare time I want to use the Scorpion for fun.
The Eagle is my current favorite, because it's accurate enough and has high capacity, allowing me to use it as the sole weapon for an Adept/N7 Fury. Overall though, I really like the Predator, even if it's the "pea shooter" default weapon, because it's such a fast weapon for fire rate and reloading, so is great with disruptor or incendiary ammo.
SMG: Hurricane, again there simply doesn't seem to be a good reason to use anything else in the SMG class.
I dunno. I want to love the Hurricane, but I don't even use this weapon in SP, and give it only to Liara and EDI since they're restricted to SMG's and pistols. I'd much rather use the Locust, because it's more accurate and has similar capacity to the Hurricane. It's only inferior on fire rate, but you can only really take advantage of that at close range, and I'd rather put a thermal clip mod than having to up the stability.
Sniper Rifle: Black Widow, Javelin. Let's face it the rest of them are just underwhelming and while you can get them to work pretty decently (I like trying the Indra from time to time) they are just in no way on the same level in terms of killing efficiency.
There's no denying that the BW and Javelin are quick-killers, but I really love the Indra. It's my favorite sniper rifle in the whole game. Long-range suppression is something I really like to do, especially in single player, and of course it's friendlier for all classes. I'd never use a Black Widow or Javelin for anything but Infiltrators and Soldiers.