Worst experience: Being cleared to land #1 on runway 29L, turning base to final (seeing another aircraft on final, probably for 29R (Runways are REAL CLOSE!), and then realizing that he was also cleared to land on 29L.
The bad part was that HE called the tower and told them he needed to do a 360, not the tower noticing it.

Other bad ones include being in the runup area, with the runway free to depart, yet being unable to contact the tower for like 6 minutes because of all the radio traffic (lady talked slow and used lots of words).

Another one was being cleared off the runway, told to taxi down alpha and turn off, and that traffic would wait for me (On tower freq). As I turned onto alpha, I was closing head on with a fast moving Global Express (That things like an RJ, and I was in a 172). He was showing no signs of slowing down and he was on ground. He rolled up to the tip of my turnoff and stopped seconds before we would have come nose to nose.

Best experience is every other day I go flying. ATC is a pretty amazing system and to this day it still amazes me. ATC deals with lots of aircraft and does a great job at keeping them perfectly separated and yet filling in the voids, keeping traffic moving, and keeping runways hot.