For me, it's kickflips for sure. I had them alright for a couple of short periods way back in the day and then always lost them (I've been a heelflipper since day 1). I put in a sold 3 or 4 hours of flatground a couple of weeks ago and really worked on the kickflips - finally got them again.
The sickest thing about relearning tricks is that everytime I relearn something, I get it way better than I had in the first place. My kickflips right now are popped way higher and level out so nicely now...and I can finally do them without having my front foot at a major angle.
I learnt backside 50-50's by popping in mid-ledge a few years back and then promptly lost them. I was totally cheating though, I'd pop my nose on and somehow pivot my back truck on and then grind - really weird. I built a nice grind box a couple of months ago and relearned them coming straight on. I still can't pop properly into a back 50 mid-ledge, but I've been practicing ollieing backwards onto a curb and it's helping.
I never really learned much in the way of transition either. I can do axle stals on small/mellow tranny but always pussy out on anything a little bigger. On the small stuff I can do other stuff like tail stall reverts and backside 50-50 reverts, but I learned that kind of stuff on little tiny, mellow 3 foot transition. All I can do on any real transition is rock fakies - sucks.