Definitely Ranma 1/2, Slayers, and Azumanga Daioh. Ranma 1/2 was my first manga series and it was a good choice, I didn't know it was so funny. XD I randomly bought Slayers one day, not even knowing about it before. It quickly became one of my favourite manga series because it's never failed to make me laugh, no matter how many times I'd read it. <3
Azumanga Daioh is just... hilarious. I took it out from the library once and I remember passing it to a bunch of friends back in the eighth grade (even people who didn't even LIKE manga) and all of them were killing themselves laughing at it. XD