I love green tree pythons although i have never had one they are one of my favourites
Not sure how good they are for a beginner i think they can be quite aggressive and don't like being handled but not positive
Carpet pythons have nice personalities generally and display nice. They tend to be more alert and aware than other python species which makes them kind of fun (turn and look at you when you approach).
But I also like some of the colubrids for similar reasons.