Well, if you've read this blog at all, you know that I'll reflexively say, Scott Rolen. But the statistics back this up--Rolen is a better third baseman than even Hall of Famers Brooks Robinson (15 Gold Gloves) and Mike Schmidt (10 Gold Gloves).
Baseball Prospectus has created a number of stastical measurements for fielding. The most important is the one called "rate," which emphasizes the number of runs a player saves at his position. The stat is based from 100--so a player with 110 saves 10 runs more a season than average; a player with 87 allows 13 more runs a season than average from his position. (There is a second measure, called "rate 2" that adjusts for ballparks and leagues.)
For their respective careers, the numbers go like this:
Brooks Robinson...107 (+7)
Mike Schmidt...108 (+8)
Scott Rolen...109 (+9)
In terms of fielding, then, Rolen is slightly more valuable at 3rd base than either Robinson or Schmidt. David Wright, the NY Met phenom, is not even close right now, with a 98 (-2) in this area. Also, Rolen and Robinson have nearly equivlent career fielding percentages (Robinson .971; Rolen .967).
Couple this together with Rolen's hitting. In their common years, ages 21-31, Schmidt's wins over replacement was 88.8; Rolen's 76.1--this marks the numbers of wins Schmidt and Rolen contributed to over these years above their average replacement. When you figure that Rolen was hurt most of last year and that this year is not yet completed, then Rolen's number is very close to Schmidt over the same period. Also, Rolen's career batting statistics (.286 BA, .376 OBP, .516 SLG, .892 OBP) measure favorably to Schmidt's (.267 BA, .380 OBP, .527 SLG, .917 OBP).
Compared to Brooks Robinson, Rolen is again favorable. Robinson's WARP from ages 21-31 was 81.1. Rolen's number will be very similar by the end of this season. By Robinson's career batting stastics are inferior to Rolen (.267 BA, .322 OBP, .401 SLG, .723 OBP). Of course, Robinson never hit more than 9 HRs over the last six seasons of his career (ages 34-40); it is hard to say how Rolen will decline after age 34.
Which is all to say that St. Louis Cardinals fans are very fortunate to have Rolen at 3rd base. In fact, one could make a very strong case that we are not only watching one of the best 1st basemen to ever play the game (Pujols), but perhaps the greatest 3rd baseman as well. What is shocking is how underappreciated he has been throughout his career, both in Philadelphia and St. Louis.