3e D&D.
I was running 2e and enjoying it a lot, had a great feel for generating power challenges that put things right on the edge without TPKs and was comfortable with running the system and throwing things in or adapting other material. I was buying lots of different systems just to read them and for inspiration for my 2e games (earthdawn, tunnels and trolls, Elric!, world of darkness sorcerer, ars magica, Pendragon, Shadowrun, Rifts, etc.). I also felt I had enough stuff to keep playing 2e for decades even if the game line ended and no new books ever came out.
So then when my preordered 3e PH came I was surprised I liked the new system enough to convert my campaign over to 3e.