A good question, and a couple things to consider... I think a major question is will you be using the AI teammates, or going at it full on solo? The AI Teammates make it far easier as they can basically be your snipers for the most part. May take some patience, but it can be worth it. If you choose to go the sniping route with the AI, another option not mentioned yet would be the new Pathfinder class. While they don't really have anything that stands out for direct action (which I'll get into), their scouting potential is by far the best in the game. They don't always locate everything, but you can easily use it to scout out the enemies, then use the AI Teammates to snipe the ones harder to reach while you snipe those that are visible.

Pathfinder aside, when it comes to active play, I'd say my favorite is either the Panther or Echelon. The Echelon is better for staying stealthy due to their class item and quick reflex ability, but the Panther's ability to throw the smoke bomb and them doing full damage with SMG's is better imo. Get yourself a Bullpup and fully upgraded it does 31 damage per hit. It has decent range and accuracy for it's class as well. If you take your time, combining the stealth with the auto mark at 25m skill, the quick reflex ability of the Echelon becomes a moot point as you "should" see most all enemies coming. For those you don't, kill them quickly and use your smoke screen and hide till they go back to unaware.

That's my two cents on it, hope it helps.