Tharja is very good, so I'd recommend doing that. Odin ranks high but in my experience, he's subpar (mine is only 4 star, though).
Edit: Playing around with my high-rated 4-star contenders for a rank up and man, Leon is actually stealing the show. I have a lot of archers already and since all archers are colorless, he was sort of at the bottom of my list, but not only is he kicking ass, but he's a canonically gay character in Echoes that's treated very well by both the writing and other characters alike. He's fucking slaying literally and figuratively. I guess I should stop caring about how many archers I already have if he's that good.
The other contender is male Robin (blue mage). I'm probably at parity with green and blue mages, so promoting him above Nino (who isn't doing as well as her ranking suggests, honestly) won't kill me. At least they aren't fucking red.