Barry Bonds and it's not even close.
His insane numbers were actually deflated due to park factors. He played 15 seasons in SF. Candlestick Park and AT&T Park are notoriously difficult stadiums to hit in. Night games in the middle of summer at 50 degrees, swirling winds, and a heavy marine layer. Good luck getting any carry on the ball.
If he got to play in those AL East band boxes with warm, muggy, summer nights, I reckon he would have hit 85 HRs in 2001.
It's true that Bonds played in a bad hitter's park. His numbers were probably deflated by it. Although he did also hit more HRs and for a higher average at home that season. I think Barry's late prime is the most incredible thing to happen in the sport. However, full context still has to be considered. It's an expansion era. Expansion twice in the 90's. This is something I think gets underrated because of the steroids, but expansion played a part in the offensive explosion of the 90's-00's.
And the steroids. I agree that the entire league was juiced, including most of the pitchers. I think his dominance of the era is actually fairly legit. It's also impossible to argue that the steroid era didn't give us inflated offensive numbers, especially HR numbers. So the raw number, I don't value as much.
That's why I have him in the conversation with Mays and Williams for second, behind Ruth. His dominance of the early 90's was great, but not as great as Mays and Williams (or Mantle, Musial, he's second tier at best, probably around #15 or #16 before the explosion in 2001). His second prime was Ruthian in it's dominance, but didn't last nearly as long as Ruth's.
Side note: the one truly good legacy of the steroid era is that it destroyed once and for all the notion that strength training for baseball was a bad idea. We don't have multiple guys hitting 60+ a year, but top to bottom, lineups are far more dangerous now than they were up until the 90's. Even if we have lost some things like real leadoff hitters. (And no, I'm not claiming the sport is clean, just not openly flagrant about it like it was)