Cecil Fielder.
Bleacher Report has him at #2 slowest ever: http://bleacherreport.com/articles/665960-mlb-power-rankings-cecil-fielder-and-the-25-slowest-players-in-mlb-history
I remember a video from when I was a kid - I think it was SportsCenter - where Fielder hit one of his 7 career triples. They showed him hitting the ball, then cut away to highlights from another game. 30 seconds later, they came back to the Tiger game to show Fielder just now rounding first base. They cut to another game for 30 seconds, then came back to see him only then rounding second. All in all, they spliced it to make it look like it took him about 2 minutes before he was finally sliding into third base. I wish I could find that video again, but I've had no luck so far.