On my own, especially if the team needs it, I'll go medic, but this is usually reserved to if I know there are good players on the team TO medic. The last few days, on a team with a pyro, 6 engies and 3 spies, I tend to not play medic at all. When this is the case, I tend to go demoman.
On the flip side, when I know I have a friend in the server who's a medic that I trust, I'll go heavy, and wreck the other team's spawn area on mesa, or just the other team in general in other maps.
And of course, I play the engineer quite often. That was what I began as, and that's a favorite. I haven't been playing him recently in live servers other than to get test data for my up and coming guide, just because of the engie madness lately, but that'll change once the rush is over.
i switch based on need or mood but i play scout, pyro, sniper most, and i also make a good heavy and i go heavy for defence or if i just think the team would benifit from a heavy, and iv also gotten better at demoman and play him alot of engie destruction or defence, and sometimes when i feel froggy i go spy, my least played class is probably soilder