Speaker (when considered in the context of his time) is one of the greatest all round players in Major League history, certainly behind Wagner,but at least in a tie with anyone else.  A brilliant fielder, with good power for the era, great speed, and a ton of smarts.  As oscar observes, his career was longer with the Indians, but he was superb with the Sox, and a key figure in their early century dynasty. 

It should also be mentioned that Speaker has the 6the highest career batting average and is the all-time leader in doubles. 

Other guys played longer for the Red Sox, and in Speaker's time, you could lead the league in homers with 9. You could make the case that, relative to the context of the time he played in, that this was the most completely well rounded of the great Red Sox players, though he actually played longer with the Indians. Williams was an indifferent fielder, Yaz was inconsistent and had a weird career pattern, other guys were great but had either some weakness or didn't sustain greatness over the course of their career. A five tool player- hit for high averages, had power by the standards of the era,, ran fast, could throw, and was not just a great fielder, but an innovative, indeed revolutionary one. A lot of people don't realize that this is one of the greatest players of all time. Speaker may very well be the most balanced great in the history of this team as well as Cleveland, where his chief rival would be Nap Lajoie.

The best center fielder not named Mays or Mantle