Well it depends on the sport really. In boxing you have weight classes which makes things a little easier to judge era vs era.

In Soccer I reckon past greats would still run today's game if they were in their prime. Guys like Pele, Maradona, Best, Ronaldo, Romario, Beckenbauer, could cope in today's era I think.

In baseball I believe it's the same because although in the late 80's through the early 2000's you had the steroid era, but hell back in the day you had higher pitching mounds, bats that were heavier and sometimes actually held together with nails, the baseballs weren't wound as tight but still you had guys who could strike out great batters or hit great pitchers...I think that skill would translate.

Ice hockey the players have gotten a little bigger but the rule changes that have been made would have benefitted the older players who were just amazing. You telling me Gretzky couldn't play today? Or Fedorov? Or Bure? Give those guys the ability to accept a 2 line pass and it would be 10-0 vs the youngsters before you could bat an eye!

In basketball guys have become a little more athletic but come on the game is not as rough these days. The stars of yesteryear could still dominate...want to know how Jordan would do today if he was in his prime? Look at a healthy Kobe Bryant from a few years ago and add a few more points. Kareem Abdul Jabaar could still dominate if he was in his prime, so could Wilt or Russell or Bird or Magic.

In Football it's a little more tricky because size matters so much, but there were skill position players who would still make huge impacts today. Barry Sanders, Walter Payton, Jerry Rice, Joe Montana, Marcus Allen, Bo Jackson, etc.