there's so many,but i chose five
1. School Rumble - when they had the school festival battle thing and Imadori was on the relay race and he decided to roll and then when he made mikoto lose the chariot race.

2.Dengeki Daisy- when Teru and Kyousuke was yelling about what they were going to do when Teru moves out

3.Skip Beat! - when kyoko was in the chicken costume and making those scary faces.I don't know why but it made me cry of laughter!

4.Skip Beat! - when Kanae was taking kyoko and hiou to her home and then all the chiildren started to attack.

5.All of Fruit Basket and Ouran High School Host Club
i have weird taste in comedy i want to hear your hilarious moments in manga

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95% of teens would cry if they saw EDWARD CULLEN at the top of a skyscraper about to jump. Put this as part of your sig if you are part of the 5% that would sit here with popcorn and a camera and yell "DO A FLIP!!!"

I can't remember much about hilarious moments of manga I've read, but now that you mention Dengeki Daisy the part that really made me laugh was when Kurosaki found out Teru was moving next to his appartment ("really far way"). That part was genius.
Recalling now, in Skip Beat! there was this part when Kyoko was visited by her former companion Sho (Shotaro) because he wanted to make sure she was the Kyoko he knew, but she still negated it, so he took out a swan (I think) and she went all loving towards it saying "Odette!" and he smiled proudly, when she fell into his trap.

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"I am a colorful fantasy born on dark nights; a delusion of black and bottomless minds; a thought that dies in the morning reborn by moonlight. What am I?"
- A Night of a Thousand Dreams

In yunohana Tsubame when Aki exclaimed that him and the main girl had done it and then his older bro is like...."a kiss won't make a girl pregnant."