Welcome to the wonderful world of bettas! Favourites come and go, at least for me. :)
I love all tail types, and to me it doesn't matter about correct form because I keep them as pets and not for showing, but I won't pass up a nicely formed betta. Right now I am into metallics, marbles, dragon and butterflies. Favourite tail types are, well, all of them I guess. Especially a really nice HMPK. I love marbles because they just change so much. All colours are gorgeous, but the more unique the better. That's not to say I won't pick out a solid red or blue betta .... because I did get a red VT boy today and he's lovely.