Of the ones I can think of off the bat, I'd probably like being a faerie. The general ability to be able to fly and being close to nature are things that sound nice to me- though I'd rather not be as tiny, it kind of fits me.
If I were to go based on my actual self though and what fits me mentally, it'd probably be a vampire, which can vary in size more and are nocturnal, which I very much tend to be. I'm also bad with the sun in general. I'm not fond of blood in general, or the sight of it, though...
"I don't know what words I can say
The wind has a way to talk to me
Flowers sleep, a silent lullaby
I pray for reply
I'm ready"
-Melfina's Song