Anything from the rebirth in the 70's through the early 90's works for me-- I have a particular fondness for the late-80's days of Psylocke, Rogue, and Havok-- but if I absolutely have to pick one, I'd go early 80's.
With the introduction of Kitty to the new team, the last piece was in place for them to be the classic X-Men I'll always remember.
Although the Astonishing line-up ain't bad either... not as keen on the new changes, but we'll see...