I think my best OC was with the Celeron300A and my Abit BE6-II - well, it was one of my most exhilarating OC's, since it was my first. I believe I got it to 450Mhz, which is 50% for those mathematically challenged at this hour of the night.
The next time I tried OC'ing was with my Opteron 165 1.8Ghz; after days upon days, I finally got it to 2.6Ghz. Then came the Core2 days and OC'ing became EASY. I got 46% out of my Core2 E6750 (which I believe is on the OC db @ [H]). Heh, went to check, I guess my best OC was with my Core2 E6400, 70%; 2.1Ghz to 3.6Ghz. Looks like I never submitted for my other Core2 OC's... probably because they were nothing special (avg OC speed, enough entries for the model, etc). I haven't really OC'd passionately since then. My i7 920 got to 4Ghz with minimal effort, which was typical for D0 stepping (50% OC).
I never really go extreme when OC'ing, and I'm almost always on air instead of water. I tend to stick to the max recommended voltages, simply because I hate killing CPUs. I've never OC'd enough to kill a CPU, but I've gotten way too lazy to try an squeeze every ounce out of them.