Two I've owned in the past that totally surprised me and kept me playing for a long time were William's Teacher's Pet and Gottlieb's Big Casino. Very hard games to complete to light the specials. Other contenders that I've owned in the past were Royal Guard, Sweethearts, and Olympics.
Two favorites that I still own are Ice Show and Melody. The Ice Show is a bit harder than your average add-a-ball, while the Melody is just plain fun. And they look great side-by-side!