I took a break from late 2001 to mid-2006, so there's going to be a big gap in this, but...
-Jeff Hardy beating Triple H for the Intercontinental Championship: I was a big Hardy fan at the time, and Triple H was one of the most dominant guys in the WWF at the time.
-Too Cool beating Edge & Christian for the WWF Tag Team Championship: Too Cool had always been one of my favorites, and this was the only time they won tag team gold.
-Randy Orton beating John Cena and Triple H for the WWE Championship at WrestleMania XXIV: Orton was awarded the title after Cena was legit injured, only to lose it immediately to Triple H. Orton won it back from Triple H at the end of the night, kickstarting the reign that saw him as champion leading up to WrestleMania. Needless to say, I didn't think he had much of a shot at retaining, so I was pleasantly surprised when they put him over Cena and Triple H in the main event of WrestleMania.
-Jeff Hardy beating Edge and Triple H for the WWE Championship: Seeing the clip of Jeff Hardy diving off the balcony through a table at Royal Rumble was what originally got me to start watching WWF in 2000. Seeing him win the most important title in the company was really something.
There are plenty of other title changes that I've really enjoyed, but those are the ones that caused me to completely mark out.