Groudon. It doesn't rely on weather like Ho-Oh, Mana, and Exca do, and removes any potential Mana and Exca do upon switching in, thus forcing them out if they don't want to fall to it. Zekrom has to rely on the shitty Bolt Strike (seriously, it misses over half the time) to get past Steel-types, and gets forced out by Groudon, Exca, possibly Gliscor, so on and so forth. I've never seen a Dialga sweep, let alone do anything threatening beyond setting up rocks against my team, and always seems to falls easily. I've had a few problems with Mewtwo before, but it's just too frail to wipe a team by itself. It's good, but Groudon is superior. As for Giratina-O, I've never had a problem with that thing either. It gave a few of my monsters a burn and stopped Deo-A from punching craters through things, but it never really did anything threatening.
Groudon is my choice as the best lategame team wiper. Not to mention that it can easily beat or stop five of the eight monsters on the list.
Edit: I take back what I said about Mewtwo. I just wiped a team with him late-game.
Last edited by Daggerfall; Nov 30th, 2012 at 4:47:14 PM.