Most of the Dota 2 players uses Steelseries 6gv2, if it's not that, then it's the Razer BWU.
However, I've noticed that some of them uses Realforce, such as InvictusGaming, I've spotted iG.ChuaN use Realforce in The International 2.
I personally think that keyboard doesn't really matter much, but it can make you play better definitely, not by instantly turning you into a pro, but just a tad bit better. Mouse on the other hand is a lot more important.