All a matter of personal preference. Unless MC goes sensei on all the girls, not just Darci (once) The Headmaster will always be my favorite. I think the kinks in this one are a lot more common than mine though (so my opinion is likely the minority, which is fine), and I think this game is very sex-forward (even dialed back a bit over time) and that is great, as the dev has said as much about his intent.
I love the story, and can't wait to see where it goes, but have to go with Once in a Lifetime, because it is finished. Probably Eternum for animations, too. The dev has even slowed the sex-forward scenes a bit in (I think) response to those games. For continuous sexual tension, this one is out front.
Full disclosure: the games mentioned are the only ones I track updates for. Played many others, including a few where the only issue was translation, but this is my full investment.