I'm usually the one for the more realistic and practical designs, and, in fact, having completed FE12 recently, I really appreciate the art style there.
Jagen's spiked pauldrons are hilarious though, albeit classic.
GBA FE art, and spritework in particular, will always be my favourite nevertheless. It's bright, it's colourful, it's easy to tell stuff apart.
If only the DS battle sprites were more unique and differed in more ways rather than just hair.