There are plenty of designs I like, some more than the one's I list. But I wouldn't be as comfortable nominating them as examples of great design as this lot.
FE7 - Pent
Pent looks stellar. I could say more... but I'm not sure I need to.
FE7 - Lloyd
If you ignore the belts, Lloyd is well put together. Cool and professional, but something about his shirt and earring betray humble beginnings. His outfit suits him perfectly for who he is: the dutiful son of a once principled mercenary, and a villain whose not actually trying to be a villain. Just a shame about the belts... seriously!
FE9/FE10 - Titania
I don't like Senri Kita's art. But Titania's design is fantastic. She looks strong, brave, and experienced, but also kind and reliable. Her armour is great: clearly feminine and plausible (especially the RD design). And the huge ponytail was a bold choice, but it works great with her distinctive colour scheme. Probably one of the best FE designs to date.
FE15 - Tobin
SoV Tobin's design has grown on me. I think I agree with everyone who mentioned him. Nothing much I can add on what's already been said. He just looks... how he should... and yet, I don't know, there's just something strangely eye-catching about his design.