I'd say he's definately top 10 - 15, maybe even higher. There are, however, always going to be question marks about him due to his suspected mob involvment. WNF did a little excerpt on his match with Ali (the one with the so called "phantom punch", where you could clearly see him being hit; they did an excellent job explaining how Walcott, the ref, botched the fight and really had no buisness being there).
I think today he'd probably be undisputed champ, or at the very least one of the alphabet strap holders. The only champ I could see really giving him trouble would be one of the Klitschko's, but then again Liston was only just over 6ft. tall, so perhaps the larger men in the HW division would be able to beat him...
I guess I don't really know how he'd do today.... but Teddy Atlas had him somewhere towards the bottom of his top 10 HW's of all time, and his oppinion is much more informed than mine.