That’s a very good question, opticalsky. Are you planning to buy your girl a title as a gift, are you going to buy one for your own collection and lend it to her, or are you going to let her try out a freely-available title before committing to anything? Are you sticking exclusively with official releases, or are you including fan translations in the mix? Would you prefer a complete standalone title, or do you think a well-put-together demo would do the trick? I’ll try and cover all of these bases, if I can:
If you’re looking for a good romance story, I can easily second Guest’s recommendation of Crescendo. To this day, it remains #1 in my Visual Novel Top 10. Objectively speaking, there are better titles out there (my number #2, Phantom of Inferno, easily outdoes it in terms of immersion and narrative depth), but Crescendo has plenty working in its favor. For starters, it doesn’t overstay its welcome; each route is just the right length and can be completed in about 3-4 hours. The shoujo-style artwork is very nice, and the soundtrack is specifically arranged to elicit a feeling of faux-nostalgia. The five different routes cover very different narrative genres, including rosy high-school romance, drama, suspense, and even supernatural themes, so it’s quite likely that your girlfriend will like at least one of the routes even if she doesn’t care for the others.
Kana ~Little Sister~ is an easy recommendation as well - provided one goes into it without any biases (a friend of mine refuses to touch the title because of his own misguided preconceptions about what the story is supposed to be about). Kana is a realistic, heart-rending drama that is quite capable of bringing you to tears. Crescendo contains an extended nod to Kana in one of its routes, so I highly recommend picking up both and having your girl read Kana first. The story has been plagiarized ad nauseum by many, many subpar VN writers, and a good chunk of people strongly believe that the original is every bit as shallow and superficial as its imitators (those of us that have read it know better).
If you’re sticking to freely available titles or trial versions, Katawa Shoujo was released to the public yesterday. I finished the demo version a few years back, and, having retraced my steps just yesterday in the full version, I can confidently say that it’s shaping up to be something special despite not having completed the title just yet. I’ll check back with you in a week or so to offer my thoughts on the story once I’ve completely finished it.
Narcissu is another title that doesn’t need much introduction. If your girlfriend isn’t too keen on anime-style paper-doll characters, this may be more accessible to her. Narcissu -Side 2nd- contains both the original story and its sequel (or should I say prequel?). The series has since gone commercial - Narcissu 3rd has been released on the Japanese PSP, bundled with retouched versions of the first two stories. The story is entirely linear, with no decision points at all; I’ll let you decide if this is something you’d like her to try or not.
The first chapter of Suika ~Wet Summer Days~ can be played as a self-contained standalone, and a very good one at that. Truth be told, the anime version of this title is what finally convinced me that hentai wasn’t just animated rape (a friend of mine from college hired me to pick up a bunch of hentai titles for him, which I watched before delivering them to him; I was thoroughly disgusted for many years to follow as a result. The first episode of the Suika anime is what made me give 18+ material another chance). The slow pacing is deliberate and those with a craving for action might get fed up, but those who love an immersive romance with a tinge of fantasy will find plenty to enjoy. The demo includes a tastefully-done sex scene, so this might be a good introduction to adult content as well.
Kira Kira and Deardrops both have exceptional demos. Easily the best that MangaGamer has to offer at the moment. I’d have already bought Deardrops if not for the fact that the game has yet to be released in English. The demos are on the long side, covering the first chapter of each game in its entirety, plus a little of the second chapter as a tease; I’d recommend Deardrops over Kira Kira since the art is more distinct and the characters are more realistic and likeable.
On the other side of the pond, we have Fate / Stay Night. A modern-day fantasy with superb characters and a very thoughtful analysis of what our dreams for the future really mean. Fate has been adapted into anime, feature film, and manga, but none of these incarnations do the original justice, if you ask me. After I’d completely finished the original story, I actually broke down in tears while discussing one of the endings with a close friend of mine. It’s a little on the pricey side to import, but worth every penny (it took me two weeks, full-time / 14-hours-a-day, to complete). There’s good reason why this sits at #3 on my Top 10.
I have plenty of other titles I can recommend, but this should be more than enough to get you started. I wish you luck, and, if you need a little more help getting your girl interested, you know where to find us.