Any variation of a heelflip. I did a switch heel and a nollie heel back in '99 just to say I landed those two tricks. Haven't tried either one since.
When I came to the realization that illusion flips weren't really frontside flips, I lost that one too -- sometime around '99 also. (Honestly, I only focus on kickflips, backside flips, 360 flips, fakie flips, and switch flips.)
When I was a kid -- back in '95 -- I watched a lot of Jason Lee, Matt Reason and Eric Pupecki footage. I remember spending hours each day practicing 360 flips. Consequently, I can do that trick pretty much whenever, but everything else sucks. I figured I might as well get a couple down well and call it a day. (Also, I have a rule: If a trick wasn't in Trilogy or Eastern Exposure 3, it doesn't exist. It keeps me sane around all the 12-year-old kids who pop nollie flip K-grinds all day...little bastards.)