There's a point in a campaign when things have been running pretty smoothly for a while, and I've been spinning out a situation long enough for things to really start getting interesting. The players have a few things in the game that they care about, and are going after their own goals. They've started to pick up non-mechanical resources. There's a fun intra-party dynamic going on. I've got a few folks I've established as nasty-scary-bad, and if things are really going well they've got a serious hate-on for a villain or two.
And usually, at this point in a campaign, one day I sit down to prep for a session and I find an opportunity: a few dominoes that have all been set up already, or a set that just need a few adjustments to really go somewhere fun. I think to myself, "If I move this thing over here, and this NPC does that, the players will totally flip out." So I polish it up, come up with a plan, and get ready.
Then, if I pull it off right, if most of my assumptions about the players and the characters turn out to be true, the ones that don't turn out in my favor anyway, and all the timing works out, there's a moment. The players realize just what it is that's happened, and they're all looking at me, and they can't decide if it's insanely evil or the most awesome thing they've ever heard of. The characters at this point are usually pissed off, often terrified, and always ready to go pound someone into the dirt. But the players are having a blast. Because when it works out just right, I've given them a goal that they could never have come up with on their own, but is still utterly perfect for their characters.
And then I've got 'em. A villain who they'll follow to the ends of the earth. A problem they'll argue about amongst each other and come up with all kinds of schemes to solve. A brilliant set-up for, at the very least, a few weeks of high-stakes, high-intensity gaming, and sometimes something that will come to define the entire campaign. When everything works out right, it's about as close to perfect as gaming gets.