My analasis of ME3 multiplayer recently.
A good vangaurd will always get the highest score, regardless. Score is relatively meaningless, its not even related to skill too much, all that matters is that you get everyone to the LZ in round 11. I'm curious just how much damage a biotic charge does....
Infiltrators are the lifeblood of the team, they are the ones who can save your life even if you die deep in the middle of enemies. Also the class that is most likely to succeed when the rest of the team dies.
Krogan soldiers. What is now possibly my favorite class, Rage + Melee is quite possibly more powerful than a vanguard, there is very little that doesnt go down in 1 or 2 hits, even less that doesnt go flying a mile after death.
Sniper rifles. These are basically the only weapon I ever use now when I use a weapon. Widow completely ignored the guardians shields, just aim at the head as you would normally and fire. Though I got the N7 sniper rifle, fitted the armor piercing and that also was decent, not nearly as damaging but a very quick reload make use for that slightly. One last point, the smoke bombs, with the sniper scope these make killing guardians even easier.
Adepts are quite powerful, biotic explosion is really the entire goal of them, warp + anything else = massive damage against anything.
Best assault rifle from what I can tell is now the geth + armor piercing mod.