Epilogue or Isolation Years. Or Blackwater Park ^^ But I really like the clean guitar in the end of White Cluster too, it really feels like an epilogue to Still Life, gives me shivers!
Wow, I cannot believe that no one has mentioned Weakness. I think that it is a very good closer, especially the way in which it ends. Anyways, I would say that Blackwater Park and To Bid You Farewell are pretty solid closers as well.
Epilogue is the coolest closer definitely. TBYF is too long for me. Weakness and Isolation Years are also great. So is Blackwater Park. It's hard to choose. White Cluster has the best ending itself.
I like White Cluster and Hex Omega because they sort of directly continue the previous tracks, giving the end of the album a lot of momentum I think. But White Cluster is my favorite just cause of the last few seconds which wrap up the album perfectly.