From the three listed.....GTP would be my #1 choise since this would be your first 'higher' level snake.

....but if you are looking for a great display snake....IMHO...you could not go wrong with any morelia because they all generally make great display snakes.....However, I like semi-arboreal carpet pythons a little more than arboreal tree pythons because they seem to move around a little more to me and can handle alot more interactive (i.e. you can handle them a bit more). They are also a little easier to care for so that is less worry you will put yourself through.

So that would be the route that I would go with (and was the route that I went with). Check out some...Diamond carpets, jungle carpets, IJ carpets, centralian carpets or if you want something a bit larger go for a coastal carpet