They all make great pets, each has there better things to offer. I prefer Versicolor above all due to kindness and color. But if you are looking for something a little more rare that also hase great color the A purpurea would be well worth the cost. The others are pretty much always available so my vote for you would be A purpurea.
Personal recomendation is the versicolor. Reasoning: none of these Ts are regularly available as anything less than small slings. Versicolors are
a) the most commonly available giving you a wider choice of vendors and pricing
b) generally cheaper than any of the others
c) sometimes available slightly larger than hatchlings because they are much more common.
This gives you a little more options in regards to pricing, and might make it a little easier to deal with if your first attempt with a sling fails. But, get what *you* want. Difficulty wise, they are all similar, and they are all attractive, distinctive Avicularia species.