Best: FE10 Sothe. For a required unit, he's actually okay. Not TOO bad, although his use diminishes over time.
Honorable Mention to: FE7's Legault, FE9's Volke
Worst: FE6 Cath. She's basically FE9 Sothe, except worse in some aspects. You already have Chad (and Astol) as a thief, so not really too good.
Dishonorable mentions towards: FE9's Sothe, FE6's Chad, Dickard Rickard in FE1, 11, 3, and 12.