I would replace Sylvia with Arlovski. I think Arlovski is more technical and more explosive than Sylvia. Yes, I know Sylvia beat Arlovski, but I still think Arlovski is the better striker (not fighter).
Penn should be up there instead of St. Pierre since he pummeled St. Pierre in the stand-up aspects of their fight. I agree with the rest of the list, Silva is probably the best overall striker in the UFC while Liddell may be the hardest hitting striking.
Cro Cop, well, he is Cro Cop. What more can I say? Edit:
I forgot about Vera ... but he is probably leaving the UFC.