For big men it has to be the 70s. Look at anyone's ATG HW list and see which decade is chokes that thing out. Below that, I'd have to say the 80s as well. For the really little guys, the last 10-15 years have been fabulous. I'm only around 40, so I can't speak for anything pre 70s.
The turn of the 20th century is indeed a fascinating era of boxing. Very much a transitional phase,as it seemed a hybrid of the old time prize fighting,and boxing as we know it today.
Being a huge fan of Ali, Frazier, and Foreman the 1970's are my favourite period. Speaking of the turn of the century - if ever build a time machine I need to go back and see Wyatt Earp ref that fight.
I would say the 70s for the Heavyweight division... Ali, Foreman, Frazier, Norton, Holmes, Shavers.....
The Golden Era --1920~1949. Less interested in pre Walker-Law era at this point anyway, and almost as interested in 50s thru today.