Here are some of my personal favorites:
Magical Diary - Very cute. Did you ever want to romance Severus Snape? Now you can. Technically, you can treat this game like the Harry Potter fanfiction you've always wanted to write or read. But even on it's own, it's a solid feel-good game (largely depending on your choices) with a charming cast of characters. Also, there are some sneaky secrets here and there which you probably won't discover in your first playthrough.
Halloween Otome - Free. The premise is ridiculous, but it's handled really well during the game. Good fun.
Backstage Pass - Ace romance option! Also one of the more interesting MCs I've seen, but I may relate a little too much to her anxiety issues. Ace romance aside, the guys are not really my cup of tea, but I love the fact that there are different career paths, friendship routes, and even a female romance option.
Nameless - I think this is Cheritz's best game so far. The dolls' backstories are interesting, then there's the MC's friendship with Soi and Shinbi (who shoud've both been romance options or at least a couple, damn it!), and the first traces of what seems later to have become the social media concept for Mystic Messenger.
C14 Dating - On my list alone for an ace romance option written by an ace writer (afaik). And there's more than one disabled character! I think the game uses its setting very well and the interactions with the characters feel genuine and lively. That said, I still think there was some untapped potential here as I had the feeling that the whole field trip scenario could have amounted to something more.
Lucky Rabbit Reflex! - Well-written game in the vein of Tokimeki Memorial. Romance routes, friendship routes, even duel endings. Warning: R-word in some instances.
Honorable mentions: The Mass Effect and Dagon Age franchises.
Patrick Weekes, one of the writers for Dragon Age: Inquisition and other Bioware titles conveniently put it like that:
To me, Dragon Age: Origins has probably the most enjoyable romance mechanics of all the games. The companion banter alone gives so much insight into the characters and their relationship with you, it's simply amazing. (I'll never forget an Alistair/Oghren banter in which Oghren asked Alistair about his and my Aeducan warden's sex life.) That said, I probably feel most connected to my Mass Effect romances, mainly my Shep's best friend and partner in crime, Garrus Vakarian.