Wow, that's hard to narrow down.
Serious:
1. The guys walk to Arlong Park with the music in the background. I still love watching that scene.
2. Croc vs Luffy when Luffy takes his foot and slams Croc's hook into the ground and breaks it. I think the first time I watched that ep I rewound that part like 5 times.
3. Luffy vs Lucci…epic. Oh and the first time he used Gear Second against Blueno.
4. Merry's funeral. I cried.
Comedy:
1. When Luffy rockets himself into Smoker and Ace. The expressions on Smoker's and Ace's faces! XD
2. Sogeking on top of the tower shooting down marines while Sanji's all "isn't our sniper great!" Then Zoro is like does he have to sing?
3. Luffy pushing the zombie back into the ground. That was hilarious.