Thanks for the replies.
primecoin, riecoin and gapcoin all innovated different useful PoW algorithms. Regarding the technical innovative aspects, Riecoin is the best (of course, I'm Riecoin's dev).
I see. Primecoin doesn't sees much development or activity. What are the differences between Riecoin and Gapcoin and why do you think that Riecoin is more innovative?, is it because it has a more sophisticated PoW function than Gapcoin?.