Even before I bought the game I loved the Pyro, so I immediately jumped onto a Dustbowl server as, yup, a Pyro. I went BLU, and first thing I did, I rushed into the corridor of 3-2, W+M1 all the way, and died in a splatter of gore. At that point I thought "Oh god, this game is too gross", but I kept playing it. And I loved it.
My first custom map was cp_labor. I thought it was an official map.
After that I went on to play duel_duel2 all the time. I don't even know what I was thinking, but I did genuinely, honestly, like that map. The cylindrical one.
EDIT: Other custom maps I loved were cp_floodzone and ctf_floodzone (I never paid much attention to which mode it was) and cp_lazytown_night (or something like that).