Number of different discussions here imo... but again, I think there is a big difference in discussion on who are the best athletes and who had the best careers.
Also agree with Mike's last point about the medal count.
Where I disagree with Mike, is a previous point he made in this thread...
50 years from now, I might say that Phelps was the best of this era, but if swimmers are smashing his times and records... I won't continue to say he belongs in the discussion of best athlete ever.
Yes, it would be plausible (and even realistic imo) to say that in that scenario Phelps would have been that much better in the 2050-2066 range if he had the training benefits of that era.... but still think you'd have to judge him for what he was.
I think that vault example Sam posted earlier in this thread is extremely valid. I'm sure Latynina would be closer to the gymnasts today had she lived in this era... but based on what is known, I don't think she can belong in any discussion of the best gymnast ever, just because she was the best of her era.
I feel that about all sports and athletes. Ruth always a big discussion point to me. Career is insane. You can easily argue his career is one of the best of all-time. I'm not willing to go with saying he would be Matt Stairs, but I don't have much / any confidence he'd be the same player today (even when factoring in the likely better diet, training, etc).
Getting this back to Phelps and Bolt.... I'd have to really look at the totality of the events they competed in. Phelps had more attempts at medals, but did he also compete at more distances... both sprint, and distance? I can't hold it against Bolt that he also doesn't run 400, 800, 1500, and marathons... golds in the 100, and possibly 200 over 3 Olympics is sick.
I guess you'd also have to consider competition. Is there better, deeper competition in the pool or on the track?
Seems awfully hard to choose between the two. Have to think an Olympics Mt. Rushmore on talent and results alone would have a great chance of including both.