Mahou Sensei Negima! - epic fantasy action, decent story + much laughs and ecchi.
GTO - stereotypical shounen comedy, lots of laughs, beats, and pantyshots. Mandatory reading.
Kurohime - action&adventure fantasy, meh~ ish plot, great art and character development.
FMA - best shounen manga IMO, amazing story, characters, plot, mature for a shounen, but with frequent laughs. Top-notch art too.
For pure comedy, theres
Yotsubato! - kinda random, but unbelivably funny.
Also
Beelzebub - very, very funny, kinda early days for it atm (24 chps), but its developing a story nicely, a unique comedy in a tired genre. Definately read.
Comedy + romance;
Ai kora - almost a stereotypical harem comedy, but with great char twists, and truly epic lulz moments. <- +1 if you liked negima.
Mx0 - unforgivably cut shot, unique and funny comedy.
Pretty Face - from the same mangaka as Mx0, don't be put off by 'gender-bending', its nothing (too) wierd, a very funny comedy.
my first serious post? Better believe it, after all... manga is;
Obligatory Loli (wouldn't be a post without it)