Many for Hobbes. Basically any time he provides clever commentary or attacks Calvin returning from school.
Calvin's babysitter Rosalyn got one in her final appearance. After previously having to resort to threats and brute force every time she babysat Calvin, she finally beat at him at his own game, literally. The moment she realized that the purpose of Calvinball is to make up new rules all the time, she immediately got into the game and eventually used it to make Calvin go to bed without any more fuss.
Calvin: Man, she picked up the nuances of this game fast!
The real Moment of Awesome in "Let's go exploring!" is that it's probably the most remembered final line in any newspaper strip.
Let's be frank, not many comic strips get final lines. And on that note, for Watterson to gracefully bow out after ten years, keeping the comic from getting stale and opening a great big gaping hole to potentially be filled by new creators, is heartwarming in and of itself.
Any of Calvin and Hobbes' arguments, especially the croquet match.
Susie inviting not Calvin, but Hobbes to her birthday party, writing on the back of the note that he "could bring that stupid kid you hang around with if you like".
Calvin of all people getting an "A" on a paper. Considering his grades?
In certain subjects like creative writing, he was nothing less than a savant.
Every time that Calvin got an insult in at Moe, usually paired with an Expospeak Gag.
Every piece of snow art Calvin has ever made.
There is one strip where Calvin asks his mom if he can buy a heavy metal album that advocates suicide and Satan-worshipping. Calvin's mom manages to convince him otherwise not by saying "no," but by sheer logic.
Mom: Calvin, the fact that these bands haven't killed themselves in ritual self-sacrifice shows that they're just in it for the money like everyone else. It's all for effect. If you want to shock and provoke, be sincere about it.
The Art Shift that accompanies any intro to 'Tracer Bullet'. Usually coupled with a hilarious, totally deadpan Film Noir self-introduction.
The name's Tracer Bullet and I keep two magnums in my desk. One's a gun, and I keep it loaded. The other's a bottle, and it keeps me loaded. I'm a private eye.