I would say emerald tree boa, i have a few and they are just amazing.
but they don't move much, even at night, they will move a little but mostly perch in a hunting position. they are however the most delicate ones, so make sure you have the right equipment and experience to take care of it, they really don't accept less than perfect conditions..
if you want one that moves way more and comes in an aray of colours, you could go with the amazon tree boa, they also spend most of their day sleeping and often in a hide on the ground, but when the lights go out , they are all over the cage.
they are also easier to care for, more forgiving if the conditions are not perfect.They have the reputation of being nippy but since you will not handle it
another cool thing is the glow in the eyes at night when light reflects in them.. pretty sweet..
of course you have the green tree python, same cathegory as the emerald tree boa, maybe a little less sensitive to it's enviroment.. nice ones to chose from.
and if you want something different, bigger, you could get a moluccan python..
i'm looking at getting one...
they will enjoy an arboreal cage since they will climb and perch.. they do get big.. 6-8 feet, best discription i could give you is they are like a reticulated python/emerald tree boa mix. absolutly demands respect .. nice snakes with amazing rainbow shine to them..
arboreals are by far my favorite
these are some of mine.. i have 5 emeralds and 5 amazon, only one green tree now always looking for more hehehe
as mentioned , thinking about getting a CB moluccan... just need to figure out if i have room for the cage first..
good luck in you decision, one thing is for sure.. you will enjoy it and most likely get more :-D
Steph