Anyone who doesn't think GSP is the best wrestler doesn't understand how good Kos was considered, Josh Koscheck is extremely decorated and is one of top 5 best wrestlers to ever get into MMA.
Up until he decided to become a striker Koscheck had the most ridiculous shoot-in to a takedown which I can remember seeing. The explosive power which he had in the initial shot and the follow through drive was amazing, he'd shoot in for the takedown, drive the guy most of the way across the cage as he tried to sprawl and still get him down with authority. Until he ran into GSP, his takedowns were unstoppable and ranked up there with the best. Then he got introduced to a whole new level of wrestling.
As for who's best, up until his fight with Vera I probably would've put Couture in a tie with GSP. He held his own against Brock and if he wanted someone taken down, they went down. But after his 3 rounds of futile takedowns against Vera, I gotta give the edge to GSP since no one's stopped him yet.