Games don't always go as expected. Sometimes things click in ways you'd never expect, and other times with all the right ingredients, things fail spectacularly.
The best session I've ever had is almost always the next one coming up. I can think back to the many many games I've played, and think of lots of good times I've had. Times my players were tapped right into the game, with everyone feeling it, but like some sort of junkie, I'm always looking to that next hit, the next time I can throw some dice and share what's in my imagination.
The worst session I ever had was the one that didn't happen. Doesn't matter which one that didn't happen. Even the worst sessions I've played in have been an experience, and hopefully I've gotten to learn something from them, even when that lesson takes a while to sink in. The sessions that don't happen aren't experiences at all, and it's hard to take something away from nothing.
Even when you can't play the game you want to play, even when no one is in the mood, don't cancel. Play something, and if you can't play something, still get together. Watch a movie, go for a walk, do something together. It beats sitting at home bummed that your game didn't happen.
This post is numbers 19 & 20 in the 30 Days of Gamemastering Challenge.