Like the OP, 3 Sisters Story is the first one I can remember playing. That was decades ago, though. I remember thinking it was basically sexy but also finding it a bit frustrating and very light on gameplay. Ended up trying a few other hentai games (True Love wasn't bad; at least there were actual choices to make and a broader story), but it wasn't until I ran into the AIF scene (Chris Cole or perhaps One Eyed Jack were the first that really hooked me) that I really saw the potential for a well-made porn game.
Probably also why I'm not all that impressed by the ren'py renaissance of late. Nice pictures are great, but meaningful gameplay with a range of gameplay choices are where it's at for me.