Pretty much any art where they stay true to their character and fit the usual art style for FE. The artists for characters like Lloyd, Hector, Eliwood, Oscar, and Zephiel really dropped the ball in my opinion.
Brave Roy is fantastic as it was actually done by the original artist for his game. The Black Knight, Tana, and Boey all have really stand out art for me as well. I'll also give an honorable mention to Reinhardt, since coming from an older game his art is much more open for changes, but I think his artist really kept true to his original look while still updating it for the current standard.
Excluding some of the more obvious choices (anything by previous official artists like Senri Kita, Yusuke Kozaki, Wada Sachiko, etc), I'm quite fond of cuboon's coloring style (wish s/he'd fix my poor Reinhardt's hand tho). I also love Asatani Tomoyo's works so far (Renais twins and someone else I'm forgetting at the moment).
Also, she may exist purely to fuck up our Klein/Brave Lyn pity rates, but Lachesis has some beautifully colored and shaded art. Too bad she sucks, cuz her artwork is borderline good enough to use her for.
Edit: oh yeah, and Tana looks amazing too. Part of the reason I'm so salty for not getting her :(((((((
Favorite Male: Ephraim
Favorite Female: Celica
I like just about everyone's art a lot, but I don't care for:
Ogma (AKA FE G.I. Joe), Clarissa (only because her hair is WAYYYY to crazy, she looks like she's trying to be Medusa with that hairstyle) Navarre because he looks kind of depressed and scared. Leon's art bothers me because he looks too stiff, Gwendolyn looks really disproportionate, Raigh just because his character is really annoying, Bartre looks simultaneously creepy and like a perve. Fae bothers me too at all times unless she's in her chicken form, Matthew's outfit is horrendous but his art's not bad, Serra is really stiff and looks like she's made of paper, Lucius is.... okay. Karel looks cool until you hear him talk. Lloyd is atrocious, probably the worst looking art by far, he looks clueless as to what he's fighting, like he doesn't know what to do in a fight. Valter looks like a Vampire but it's also kind of funny. Chrom's art is good, I just feel like they should have made him look a little more muscular. Odin's look really bothers me, but I think it's his character design more than the art. I HATE those sandles. Oboro's facial expressions annoy me. She could have been waifu if she knew how to look normal.
That's...a hard one, to say the least. There's too many to consider, but at the very top of my head, I can definitely agree HACCAN is up there (he can make Hinoka hot for fuck's sake). Other than that, I really really like Charlotte's art. It's just perfect.
Honorable mention to kippu for making their kawaii drawings in Ninian, Linde & Elincia.