I've been to hundreds of ball games in my life so to narrow it down would be difficult.
Heres my list of the best players I have seen in no particuliar order:
Mantle (childhood hero)
Mays
Aaron
Rod Carew (best hitter)
Tony Gwynn (2nd best)
Tom Seaver
Clemans
Mattingly
Rose
Mariano Rivera
Let's see. The last game I went to:
Barry Bonds - I was standing in right field waiting for the ball to go over my head into McCovey Cove. Barry popped out in foul territory. It so confused the baserunner that he was doubled off. What, no home run?
Roger Clemens as a Yank vs. the A's. The A's teed off on him that day. I remember an asian girl that didn't know baseball that came that day asking me "He's supposed to be good?".
Greg Maddux pitching against the Giants at Candlestick. Almost froze to death. Had to leave after six innings.
Al Kaline of the Detroit Tigers. One of the best outfielders ever, a childhood hero of mine. Hall of Famer too.
Derek Jeter of the Yankees. Maybe not the best player, and I hate the Yanks, but I sure would like to look like Derek in those pinstripes.
Many others, too many to mention.
Al Kaline, man I followed him every day in the papers...