Here are my top 4 and why:
T206 Honus Wagner - the most important card in the history of the hobby.
T206 Ty Cobb (red background) - maybe the most popular and sought after pre-war card.
T206 Walter Johnson (portrait) - most frequently voted the best looking card ever produced.
1952 Topps Mickey Mantle - THE definitive post-war and most recognizable card in collecting.
I completely discount any votes for cards from ANY other sport (esp. basketball, hockey and soccer????) than baseball. It all started with BASEBALL card collecting and will ALWAYS be #1 hands down.
Feel free to disagree, that's what this forum is all about!