My 1957 Topps Brooks Robinson Rookie Card. I got it pretty cheap off of ebay (I guess because it was off centered, I didn't care). It is by far my most favorite card.
Honorable mention would have to go to:My awesome 1970 Topps Nolan Ryan- I wasn't even taking building this set seriously until i saw this card listed on Ebay for around $40.00. I ended up getting it for $61.00 (after Mallory convinced me to bid on it). Now I am getting closer to finishing the 70 set... AWESOME!