I really LOVE the Mirasol T'ika yarn, it's pima cotton and soft and feels really nice. It also doen't hurt my hands to work with like most cottons do. That being said... it's kinda pricey and you only get 91 yards of DK weight yarn per skein, so probably not ideal for an afghan.
Makes me wonder if there's a nice worsted pima cotton that isn't too spendy.
Ravelry linky