While I was writing this with the help of the always awesome Baseball Reference, I discovered something that I sort of knew, but has been confirmed.
According to WAR (Wins Above Replacement), Alex Rodriguez is now the greatest Yankee third baseman of all time.
A-Rod didn't have a great year by his standards, though it was still a very good year (.270/.341/.506; his .847 OPS is 111 points below his career average; his 123 OPS+ is 22 points below his career average). According to WAR, he was 2.9 wins better than a replacement-level third baseman (3.6 wins better for his offense, but 0.7 wins worse for his defense).
A 2.9 WAR is considered a good season (it ranked 80th among all Major Leaguers this season), but it's the lowest of A-Rod's career since he became a full time player. He's been that good.
Anyway, A-Rod's 2.9 WAR this year pushed his career total to 101.9 and into 20th place all-time. This year he pulled ahead of Cap Anson (99.5); next on the list is Joe Morgan (103.5). (Everyone with at least 76 WAR is in the Hall of Fame.)
But he's only been a Yankee for seven seasons. As a Yankee, he's piled up 40.9 wins above replacement, ranking him 0.9 behind Phil Rizzuto for 17th on the list.
But those 2.9 WAR this season leap-frogged him over Don Mattingly (39.8), Gil McDougald (40.0) and Graig Nettles (40.6), who until this season had the most WAR of any third baseman in Yankee history. Now that title belongs to Alex Rodriguez.
He's still not a True Yankee, though.