I finally unlocked every class tonight. I'm still missing the Black Widow.
My favorite class / weapon setup has to be the Geth Infiltrator with my N7 Valiant sniper rifle, combined with damage and piercing mods. I can see enemies through walls, I can shoot them through light cover, and I can turn invisible to escape danger / give myself a massive damage boost.
So, what about you? What class and weapon setups have you found to be your favorites?