Lorena is, I'd say, the best if you could bring only 1 character. Frozen Rampart and Flash Freeze are just darn efficient and she's otherwise a good jack of all trades. However, she also doesn't have very good synergy with the rest of the team (which I think fits her character kind of well).
In most group battles your most important unit is your tank, so Alexander and/or Cynthia.
The weakest is probably Ligea. Not because she is weak, but because she is the one most easily disrupted.