If you want to give it to Clemens, I guess you could do it based on longevity. However, in the time he was in Boston, Pedro Martinez was easily the best pitcher.
As for Cy Young, you do know the stat of ERA+ compares ballparks and the eras the game was played in, correct?
[table]Pitcher|Career ERA+
P. Martinez|160
R. Clemens|143
C. Young|138[/table]