I know it sounds crazy, but I think the old M-4 Shuriken is one of the more underrated weapons out there. It tends to get overlooked with all the shiny, fancy, N7 stuff, but I think the old girl is still a great weapon for power-oriented classes, or as a contingency for infiltrators who get an indoor close quarters type map through random. I think the damage can be made formidable if you hit that "sweet spot." I like to start it at the mid-upper chest and let the recoil take it to three or four shots through the face. Especially on the classes that get bonuses for headshots, I think it works well. It does well on shields, health, and barriers, plus I usually get a stagger from 6 rounds to the upper torso. The weapon is REALLY light (even without U.M.) and ammo is usually a non-issue. (I think you get like 400-500 rounds or something like that) it feels like it gives you a much more manageable recoil than some of the other burst weapons.
All around, I LOVE the M-4. That's a burst weapon I love to death, and its a standard sidearm on about 80% of my classes. It fits almost any playstyle. My favorite attachments have to be for high velocity barrel (take that, guardians) and a heat sink so I can fire, fire, fire...