I like Dominant 7th chords. They are cornerstones for the styles of Blues and Jazz, and they pop up everywhere in between. And they are different. Major 7ths and Minor 7ths conform to the Diatonic Scale, but Dominant 7ths are just too cool to hang out with that crowd. Theory-wise, a Dominant 7th chord is essentially a Major Triad that stole a minor 7th and got away with it!
So welcome to the land of Dominant 7ths!!! There are many more but this should give you a good head start:
(note: many of these are inversions so look carefully before memorizing)