Wow, this is hard...
Best: Caeda (FE12). Runner ups to Heather, MIST(Both), Elice, Fiona, Lyndis, Astrid(FE10), Elincia(FE9 Ending &FE10), Leanne(FE10), Priscilla, Norne, and, of course, MARIA.
Worst: I have to say Dorothy, despite that she is one of my favorite characters. Runner up to Ena, Niime, Lena(FE11&12), Nyna(FE11&12).