If you really want to scorewh0re on bronze/silver, Vanguards are the only way to go (Slayer, Drell, or Asari, esp). Flamer Trooper can't get in Flamer range before the other players kill the enemies. Vanguards destroy the whole spawn before other players even know where it is. Or so I've heard...
Depends on the settings and others -- I had a particularly high-scoring Bronze round as the Demolisher + Grenade Cap IV against Geth on White, IIRC, where none of my teammates used grenades so I could pretty much lob arc grenades at every spawn while making a tour of boxes. Just under 81K, IIRC, which is a score I can't yet claim to have reached on silver. Have also seen flamers do pretty well on Glacier.
But if you're going U/U/B, a Vanguard is probably indeed your best bet for score-wh0ring w/o consumables. You might always draw a map with longer sight lines and more walking and climbing required. Been completely pointless as a Krogan Warlord because almost everything's dead before I can get in hammer range, and if I'm not going to be able to use the hammer I might as well have chosen a different class.
As for the thread topic... I dislike playing Krogan (although granted, I don't have the Kroguard yet) for their (lack of) speed and how much their bulk blocks my line of sight c/o the obnoxious camera positioning, can't stand Hunter Mode's overlay, and find Turians somewhat uninteresting. TGI Harrier is a stronger kit than a Volus Adept with a suppressed pistol, but I genuinely enjoyed the latter more than the former.