I've kept all 3 of those species and have plenty of experience working with them, id go for a carpet python, they are much more active, have amazing colouration (and theres a lot of growth on the Carpet morph scene atm) and feed very well. They can be a tad nippy when young but tame down very well.Its either a ball Python, red tailed boa, or a carpet python.
What would you choose?
Royals are nice snakes but can be very fussy feeders and arent very active at all. and most of the morphs are expensive, Pastels and spiders are nice entry level morphs price wise.
Boa contrictor are very nice snakes, and make very nice show animals but they are potentially very large snakes, i currently work with a 5-6' female who weights a few Kilo and she does have a good temperament but you need to be aware that they can potentially cause you bodily harm. I also find them to be similar to royals in activity levels, young BC are fairly activie but as they grow they tend to sit around most of the time.
But Ultimately its what you want out of a snake.