Anything with biotic power would be a good choice, since that's what Garrus and Tali both lack. Engineer would be redundant, so I wouldn't recommend it. Delspencerdeltorro 01:06, October 28, 2011 (UTC)
If you're going for a balanced squad, a biotic class like Adept or Sentinel might be your best bet; Garrus and Tali are a tech-heavy squad, good against mechs and shielded enemies (though Garrus' AP Ammo power gives him an edge over armored enemies as well), so a class that has access to Warp (Adept or Sentinel) to punch through armor or barriers might be the way to go, but the damage bonus SMG's and shotguns get against shields and barriers might make Vanguard worth a look, too.
If you're going to be Vanguard or Engineer, you should think of squad ammo powers from your squadmates. Vanguard lacks Disruptor ammo and should take Zaeed on geth missions. Engineer also lacks Incendiary ammo and needs Jacob or Grunt to make up for that.