I think the best eras for DC were...
1. early Silver Age. from the creation of Barry Allen to the forming of the Justice League. I feel that conceptually that era essentially created The DCU we know today.
2. the Mid to late '80s. Watchman, Darknight return Crisis on Infinite Earths, The killing Joke, The Judas Contract. I feel this was when Dc's writing was at its best.
but my personal favorites are the late bronze age (new Teen Titans, Great Darkness Saga) and the post-death of Superman DC in the mid to late '90s.