Hmm...
As far as guys like Khali and Gonzalez go, I agree with Greg that they were more attractions than wrestlers. They were in there for their size, presence, and aura... end of.
As for my personal pick for "worst wrestler" of all time? I'm sure I can go through the annals of history and come up with a number of jobbers (particularly during the 80's) that served as nothing more than a warm body for the then-current "Superstar Du Jour" to beat up on. A fair number of these guys were low skilled, significantly overweight / out of shape, and weren't too many degrees above canvas fodder.
I understand though that that's not really what this thread is all about. Perhaps a more fitting title would be "Who's the worst well known wrestler of all time in your opinion."
I'm not sure if he's number 1 for me, but Larry Zbyszko immediately springs to mind as a guy who I have no clue as to how he got to where he was. Perhaps it's because I missed his heyday in the AWA, with my only real exposure to him being in WCW. Nonetheless, I always saw him as a mouthy but boring, competent but limited, hints of color but mostly vanilla competitor. Nothing about him ever screamed "main event" to me, let alone anything that would drive him into a top card program. Yet there he was, taking up TV time and wriggling his way into (at least) mid-card spots. His commentary was only slightly more entertaining than his in-ring presence.
Another competitor who I'd classify almost identically is Rugged Ronnie Garvin for similar reasons.