As far as species go, I've only had experience with Splendeds so...
My favourite tail type is definitely a rosetail. They are so elegant imo. I like all tail types, though...not picky in that aspect. More choosy when it comes to colour.
My favourite colours are something with black or white. I have a soft spot for purple females, and I love dragons and butterflies too.
For me a betta has to 'click' more. But there are some I don't like, such as solid reds and blues (or a combination of the two). I am also not a very big fan of Cambodians.
Puck (Yellow koi HM king) / Scar (White HM) / Salt 'n Pepper (Silver HMPK) / 'That fish' (Copper butterfly HM) / No name (White DT) / No name 2 (Turquoise/white marble HM) / No name 3 (Lilac HM)
Bo Peep (Cambodian VT) / Goldilocks (Yellow VT) / Joie de Vivre (Multicolour VT) / Lil' Peep (Blue VT) / No name (Dalmatian VT) / No name 2