Too many moments to define a favourite.
Tele-traps on people pissing me off is good, I once got a heavy stuck in the teleporter, told him to stay there and defend my stuff... and he AGREED. And next round wanted me to do it to him again, so I did, gave him a dispenser and he camped the point the entire game and didn't do half bad.
Skywalking is amusing, there was one time where a clan were pissing me off by being general douchebags, so I got up there and set a sentry on the roof of the world. Didn't let them out of spawn. Had to stop doing it though when some innocents joined.
Every now and then I manage to get my entire team to spawn camp as engineer, just dancing, while I play cotton-eye joe. Always amusing, even better is when we move onto different classes. E.G. Heavy with KGB taunt while playing Eye of the Tiger.
As for actual playing, it's much harder to pull of than it was back in beta, but backstab chains of 8-12 weren't uncommon for me. People just sucked so much harder.
Flying whiskey shanks are good fun, especially when I get 3-4 people doing it, there have been times where me and a buddy did it for the entire game, totally dominating the sniper ledge on 2fort. We'd just pipe bomb jump at snipers with our bottles out. One of us may get sniped but then there will be 3 demomen with no health to finish the job, (or die).
Using bonk to kill engineers with their own sentry is great fun, I recently got it to work and survived by hiding behind his dispenser, taking out all his stuff and taking the intel. Similar to this, taking out sentries via circle strafing while batting it down is rather humiliating for the sentry. Favourite moment of that was when I got it to work in sudden death with just me and the engineer left. (He wasn't at his stuff).
So favourite TF2 moment? Too many to name just one.