The longest, deepest, coldest, strongest current, lowest visibility, my beautiful dive buddy's 100th dive (you know how #100 is supposed to be done!), my beautiful dive buddies, all the famous people I've dived with, all the newbies I've rescued (even if their boyfriends didn't want her to be rescued), my equipment (especially my tattered wetsuit with about 600 dives on it), my SAC rate, the power in my German legs, the number of different species I film on a dive, the dive with the greatest number of close shark encounters...
Brag? What's there to brag about? I think the best thing about chatting with other divers is to try to give them some new piece of information (in my case about marine life) that will enhance THEIR diving experience.