I was always especially fond of V12. Bravan is one of my favourite recurring subseries in Viper, though this is still before that Kuwagata chap became the sensitive soul we all know and love(he's still a high-pitched monster-rapist here).
Angel Dust is likewise one of my favourite parodies, they managed to develop the two heroines in an endearing manner in a short time and the sex scenes are quite nice. Yuria seems to get more rude screentime though, which is a shame because I prefer Kate.
I'll confess that I never touched the third game, though.