Alma - I think it's kind of sweet. It's soft and a bit exotic feeling. I like the meaning a lot, too.
Alva - This is more masculine to me. On a girl it feels kind of... frozen and pointy. Interesting, but not my favorite thing.
Alba - This is pretty surnamey to me since that's the only way I've heard it. It sounds more masculine to me, too. I can't really imagine what I'd think of it on a girl.
These also all make me think of
Alta, which was my husband's great-grandmother's name.
I like
Alma best, easily.