Manu good ones already listed i pickef a couple i haven't seen yet. Of current American cartoons i think one of the best running is Gravity Falls. I really liked the green lantern series, sadly short lived, Grey Delise did a great job on aya's voice. I actually talked to her on twitter and she did my voice mail in thst voice. She does this for charity now and then. Batman the animated series was really good in the 90's.

Thundercats and Galtar and the Golden Lance where two of my favorites.

Ren & Stimpy Scooby-Doo South Park

Calvin and hobs

Pretty much anything that played on the old Boomerang, before it got all "New CN-ified".

I used to love Zoids but I think Dragon Ball Z wins overall favorite. Edit: just noticed you said American. Oops.