My vote goes to Astrid.
I've never found Geoffery to be anything special except for his paragon which I yank the first moment I can.
Oscar(along with Kieran)got beat up by the nerf stick inbetween games.
Fiona certainly has lots of potential, but unless you are going to boss/priest abuse a lot, she'll likely never catch up with the rest of the characters in her group.
Level isn't too big of an issue for Astrid because of paragon. Feed her a few kills and she'll jump up a few levels easily. Plus, other than Marcia, the other units she is initially around don't really scream awesome, so she isn't really competing against bunches of characters for exp.