Around 2000
Mark Calaway's best years as The Undertaker were 2000-2010 in my opinion.
Anybody saying attitude era is an idiot. He was pushed before and after the attitude era, he fell off during the attitude era in favor of better stars like HHH, The Rock, and Stone Cold. After there was a brand split, there became a lot more opportunities and he started to feud for the WWE title on SD and become a top guy again. His WM appearances starting to take traction and his foundation as a legend started to take appearance. So ruthless aggression era.
I'd say 2004- early 10. He had some of his best matches during this time vs Shawn Michaels. Batista and Edge and he showed even at an older age he could still add new moves to his set ex. Hell's Gate. His ring adaptation during this time showed he was one of the best even in his 40's.
After the Attitude Era, maybe. He's always been in prime condition though. The way he looked at Wrestlemania 31 and the way he looked at Wrestlemania 20.. almost the exact same.
Attitude
In my opinion, 1997-2009. Those were his best years.
Ruthless aggression
best work was probably 2000-2013
during the middle ages
