My favorite (if I have to pick only one) is Gymnogeophagus balzanii. I love my Humpheads!
My wife's favorite (I was really stoked to hear she has one!) is Astatheros robertsonii. The blue spangles and dark base color make them one of the most beautiful cichlids out there.
A few more that I really like:
Tahuantinsuyoa macantzatza
Geophagus steindachneri
Geophagus abalios