There isn't a heavyweight right now even remotely close to being able to defeat Klitschko. Manny Steward has transformed this guy from a heartless waste of talent to one of the better heavyweights of my life.
How great Klitschko really is ... we may never know. There's no one out there to challenge him. A fight between Klitschko in his prime and Lennox Lewis in his prime would have been a great thing to see. Both huge men, very skilled, with absolutely lethal jabs.
The only guy out there even remotely appealing to me as a Klitschko opponent is Sam Peter, who Wlad would probably tear through as well. Hopefully that fight happens, and then some time in 2008, some new up and coming heavys emerge that can give Wlad a good fight as he winds down his career.
Ring Magazine Top 10 Heavys right now

...
1 1 110 WLADIMIR KLITSCHKO 48-3 (43)
2 2 113 SAMUEL PETER 28-1 (22)
3 3 47 OLEG MASKAEV 34-5 (26)
4 4 33 RUSLAN CHAGAEV 23-0-1 (17)
5 5 81 NICOLAY VALUEV 46-1 (34)
6 6 5 SULTAN IBRAGIMOV 21-0-1 (17)
7 7 66 SERGEI LIAKHOVICH 23-2 (14)
8 8 164 LAMON BREWSTER 33-3 (29)
9 9 5 CHRIS BYRD 40-3-1 (21)
10 10 5 VLADIMIR VIRCHIS 23-1 (19)