From what I've heard Sumatrans (Curtus) are the most docile, then Borneos, then Bloods - although none of them are that aggressive unless you keep them too hot.

Bloods color up as they grow, but good genes makes a difference too. The best way to get a redder animal is to check pics of the parents or ensure that it comes from a known high-red bloodline.

I have a carpet now (Diamond/Jungle) and am planning to pick up an orange-head Sumatran soon.