I think assassin, as his gun, Jackal is 100% accurate and when paired with a laser scope it is unbeatable. Well, thats for me anyway. I did not die with a level 23 assassin with a Jackal and scope on the final level.
Assassin with a good sniper is great for picking the enemy off 1 by 1, but then Commando I think is great against groups with landmines, explosives and heavy machine guns! So I think those 2 are best
I use the tank just because he has awesome health. Then I use a high damage reducing shield like the meat shield and a good pistol. The p99 can kill in 2 or 3 shots. Plus the fast healing makes you invincible.
I used the Tank for practically most of the campaign but the sniper for the last level as I just hid at the back of one of the small ships and hit the boss in the back of the head. I like the tank as he can take lots of hits and I <3 shotguns but the assassin is good with cloak, wait till someone passes then use a melee weapon (which in my mind should be secondaries) then use the smoke to move away from near bye enemies.
i like medic, idk why, probably because it's balanced, the killstreaks kinda suck though and the best for this class i think is the 'Dragon' powerful, accurate, and high rate of fire (just click the mouse button rapidly)
I used Commando for half of the campaign and the other half I used assassin. They are very helpful on the last battle against the boss because you can just snipe his head with the Jackal. Well that is what I did and it was fairly easy. I did also beat the whole campaign with the medic. They are not so good at start but they come in handy at the end. Tanks I don't really like because of their guns and shields but I could understand why so much people like them and beat the campaign with them.