For me, it's Ron Santo. (although I would also agree on Blyleven, Andre Dawson and maybe Jack Morris.)
When he retired, he was number 2 on the list for home runs by a third baseman (further down now, but the stats should be looked at in the era in which they were achieved, not when compared to the current era of small ballparks, pitchers who aren't allowed to throw inside, and "chemical enhancement".)
In addition, he was a nine-time All-Star, and won 5 Gold Gloves.
Compare his stats to Brooks Robinson, and you will be amazed. I will agree that Robinson was the better third baseman defensively, but I don't think the difference in fielding ability is enough to make up for the fact that Santo was a much better hitter.
Unfortunately, Santo has a few strikes against him.
First, the current Veterans Committee seems to be in no hurry to induct anyone. My thought is that they feel their accomplishments will be somehow diminished by having more players in the Hall.
Secondly, Santo was not all that well liked by most of the players he played against. Certainly his famous heel clicking didn't make him any friends, and I think he actually felt that he shouldn't be friendly with the other team.
Finally, I believe Santo has been unfairly hurt by the lack of success of the Cubs during his career. I think perhaps the voters feel it would be very tough to have another member of those 1960's and early 70's Cubs in the Hall when they never won a division, pennant or World Series. (Banks, Williams and Jenkins are in all ready) I guess they can't fathom the idea of 4 teammates all being in the Hall without at least some history of winning.
Quick note to some of the above comments - Bonds is NOT eligible for the Hall yet. Providing he does not make a comeback, he will be eligible in 2013.
Mike Mussina also has not been retired long enough. He will be eligible in 2014.
And sorry, but Pete Rose is not eligible either. And probably never will be.