In my lifetime, it was clearly Bonds. I know everyone beats him up over the steroids, but he was a hall of fame player before that. And he carried the pirates to 3 playoff series, and with a little luck, would have been in World Series with them. I saw him play many times, and I bet if you asked pitchers in his day, they would say he was the most feared.
I agree about this, in my lifetime. I watched (mostly) all of Bonds' games with the Giants in his peak. He'd get maybe one good pitch to hit during an at bat, and he'd smash it out of the ballpark. Steroids, yes, he did, but no one has come anywhere near what he did and so many of these guys today use some performance enhancer or something anyhow.