I don't know if I have an absolute favorite since I love a variety of ecchi and fanservice with them asses so here are some of my favorites. There are probably more, but I can't remember at the moment lol.
Gacha Gacha. The artwork was nice and very ecchi
This img is from the second spinoff series, but in terms of story, I liked the first one better.
Mouse. A little corny, but still a favorite of mine (^^)
Eiken. One of the worst series ever made, but I still liked it somehow lol.
Godannar. I hated yet loved this series.
Usagi de Cue. Watched it because of the fanservice and ears.