Easily Charles Barkley. To be 6'6 (and listed as 6'4 in some places), under the tutelage of Moses Malone, to come out and dominate the PF position (in the late 80's to 90's era, one can argue that nobody was better except Karl Malone, who had much better guards feeding him) is simply amazing.
True here, Barkley was surprisingly amazing as a PF depsite his size. Thing is you'd see him block and throw down on 7' dudes regularly. And he was the leading scorer on the Dream Team. And just a mean dude. There is a lot to like with Barkley and you can see how people probably underestimated him coming up. Once established, he's a guy you'd want nothing to do with though.
I'll vote for Rondo, too. Does things that are absolutely mind-bending, and drafted at #21.
I was actually a little surprised at something. Took a look at this list of "The 20 Best Undrafted NBA Players"
http://bleacherreport.com/articles/410608-the-20-best-undrafted-players-in-nba-history
Figured that somewhere along the line, there would have been some superstar-type that had slipped through the cracks. Lots of great players on the list, but mostly "Uber-Role Players," like Bruce Bowen and Ben Wallace and Udonis Haslem. Maybe I'm just used to the NFL, where it seems that there's ALWAYS someone who went undrafted that becomes one of the best players at their position.
EDIT: I'll have to save Ginoboli. Almost went undrafted (#57), and when healthy he's a helluva player.
Another good one. Rondo has played so far above my expectations. I saw him at Kentucky a few times and I figured at best a good ball handler and defenders... the offensive mind, passing, and slashing ability was not something I saw coming, nor the crazy ball fakes he has added to his repitoire. And he has become an absolute thief, roaming the floor off his man he is so dangerous. Like you say, he surprisies me with a move every once in a while that seems unlike any I've ever seen.
And Ginobili is occasionaly as good as any SG in the league, so yes that's definitely a good one. Like Kobe, he can be one of those guys who, playing against your team, you're thinking "please don't make this shot" and he does. And he keeps making them. He has always played hard and been clutch and kept his head down through it all. Health issues for sure but healthy he is quite a player.