For comedy I'd recommend Hayate the Combat Butler. It is very funny, and if you are already a fan of anime/manga and gaming it is also very referential to that culture for tons of jokes that may be missed by others.
If you can find a copy of the Azumanga Daioh collection it is very amusing. It is a comic strip style collection not a typical "graphic novel" style manga, but still, very funny (as is the anime.)
Also, for a romantic comedy I'd go with Love*Com which is a very funny read as well as sweet.
For action, fantasy and comedy all mixed together (along with a bit of romance and a harem) I'd recommend Rosario Vampire. It starts off a little bit slower, but currently up to volume 7 is released and it is definitely picking up steam.
Another good fantasy/comedy story is Gakuen Alice. It is a great read and has enough mystery to keep it interesting. Don't let the fact that it is about elementary school kids turn you off it.
Bleach is a really good choice for some good action and the story (unlike the large amount of fillers that appear in the anime) moves along at a pretty rapid pace after the first few volumes. Tite Kubo's first manga Zombie Powder is also decent, but, though it technically "finished" after 4 volumes there is obviously a lot more story left to tell (I believe it was cancelled due to not being popular.)
There are also a ton of great romantic comedies I could mention that tend more toward the typical shojo drama at times, but I won't throw them out there in case you aren't interested.