Defeating Gnasty Gnorc in the first game will feel this way to most players in their first playthrough.
The first time you successfully ace a speedway may also count.
The fight against Ripto in "Gateway to Glimmer" (name may vary depending on your location), in which you both fight for MacGuffin powerups and proceed to fight it out on a platform, at which point he will create robitic versions of Gulp. Then after that Ripto will sink the platform you're fighting on and jump on a cyborg bird- you promptly get a powerup and give chase, dog-fighting all the way. To say nothing of the mini-games that follow.
The End of Gateway to Glimmer. Moneybags refuses to give back all the money he's taken when Elora tells him to. Elora issues a calm command. Hunter sits on him. Crowning moment for both of them.
Each boss battle in Year of the Dragon is amazing, but both ones against the Sorceress are absolutely fantastic.
Scorch was pretty amazing. He wasn't nearly as hard as Spike was, but he did live up to being the Monster to End All Monsters.
The skateparks are all pretty cool, and getting the course record feels damn cool.
Hunter boasts, but he really is a good skateboarder as is obvious the first time you try to beat him before you've gotten the controls worked out.
Hunter's manta-ray from the second game returns, and you go around hunting the metallic sharks that also returned. Laser-Guided Karma at its absolute finest.
There is one where you can play as Sgt. Bird and fly around Enchanted Towers, giving a Curb-Stomp Battle to any enemies in your way, while completing the regular quests. (This option will likely be available after you've completed the level the first time round, though.
Super Bonus Round from Year of the Dragon. Full stop.
Sheila, Sgt. Byrd, Bentley and Agent 9 get one each upon being released by teaching Moneybags a lesson in their own unique way; kick, rocket launcher, club and laser gun respectively.
At the end of the third game, you get to chase down Moneybags and burn him until he drops all of the loot he took from you throughout the game; then force him to give up the dragon egg.