I agree to a point with Alex Rodriguez. The combination of hype and his huuuuuge contract make him, in most peoples eyes, a very over-rated ball player.
With the exception of his defensive play at SS, his stats will tell you differently though.
When he played SS Rodriguez was an average defender. Money is also not that big of a deal with the Yankees. If this was a sport where there was revenue sharing and a salary cap his salary might be held against him, but usually I don't even do that since teams usually decide to pay players that amount and most good players will eventually make big bucks anyhow. If somebody that was a lock for the Hall of Fame said he would only take a million dollars a year for the entirety of his career when his last contract ended and he was in his prime so the team could go out and get a bunch of superstars with the extra money, I might give him a big bonus for that. I have also never seen anything like that happen...ever.
I didn't take into account whether McGwire did steroids just performance on the field. It wasn't against the rules at the time, and if we go through all the scenarios where one person has an advantage over people of another era everybody that has ever played would have a asterisk next to their name Examples in baseball of this would be:
Until the latter part of the 19th century pitchers weren't allowed to throw over handed.
They allowed the spitball until about the 1920.
At one time fielders weren't allowed to use gloves and when they did start they were a lot smaller than today.
Blacks weren't allowed to play form 1880 until Jackie Robinson.
Baseball was the dominant sport in America until the 1960s when football took over and the best athletes went there and also went into basketball later on
They didn't have top of the line legal supplements until about the 1960s forward and people before that didn't have nutritionist, expert physical trainers, and the modern gyms like they do now.
The people in the first half of the 20th century didn't have steroids to help them.
They didn't have motion analysis and video a long time ago to help analyze yourself and members of another team.
Ballparks are smaller now.
In the deadball era balls didn't have as much bounce to them and they let the pitchers mound be a mile high.
The best non-American Latino players didn't get into the game until the latter half of the 20th century.
There is medical advancement like Tommy John surgery that can bring players back from injury that would have ended careers before hand.
As you can see almost all players have both advantages and disadvantages over almost every other baseball player in another era.
That's just with baseball, with football the rule changes throughout time make it a lot worse in comparing across eras.
I did go back and found somebody that might be better at first base (depending on if you want to count him as a first baseman or not) than Mark McGwire. Frank Thomas best years match up pretty good with McGwire's and you could make a rational argument that he was better, again depending on if you classify him as a first baseman.