This honestly. It’s when the books were most exciting to me. Probably everything from X-Tinction Agenda through AoA, or maybe through OZT which was just after AOA.
The Morrison New X-men era is probably a close second, followed by the Messiah War trilogy period (Messiah Complex through Second Coming), then the Outback years and finally right now as we get to the second part of Reign of X with adult Cable having just come back.
So those are my Top 5 in order.