Easily Cynthia (this is excluding Rematches, for me. I haven't got HG or SS, but I know Lance's rematch team is very strong). For me, the order is...
Cynthia
Green (Rival)
Wallace
Steven
Lance (in GSC, and first time round in HGSS). But.. Yeah, I've explained already.
The Rival can be a real toughie alot of the time, so although I put him second, he's very close to Cynthia. (intimate, ooh... Lol)