It's funny how so many of the best visual novels that initially appear to be "dating sims" turn out to be anything but. Ever17 is, like many other visual novels, built around a series of "routes" devoted to romancing different girls, but don't let that fool you. The characters are all likable, true, but what makes the game arguably the best visual novel ever localized into the English language is the incredibly complex mystery at its core. Judge the game by any of its individual routes, and it is a reasonably entertaining but hardly revolutionary science-fiction thriller, with a couple of absolutely brilliant twists and a knack for being unpredictable.
It's the final route that launches it into the stratosphere. I'll tell you now—if you're interested in playing Ever17, you should not be on this TV Tropes page. Go find it, play it and when you're finished, come back and realize how easily you were duped. There are plenty of other visual novels that have mind-blowing final routes, but not many are written as intelligently and consistently as Ever17's is. Not only does it make perfect sense considering previous developments, but it takes full advantage of the medium in order to tell its story. It pulls off tricks that literally could not have been done in any other art form, and it does so without cheapening the story or angering the reader. It's quite a feat.
I wouldn't say that Ever17 is my favorite visual novel: the first few routes suffer a lot in comparison to the final one, and I would have preferred it if the game's end wasn't so ridiculously happy. But Ever17 is a fan favorite for a reason—it's a game that should be played by anyone with an interest in the medium.
(Additionally, Ever17 is completely work-safe! So if you're avoiding visual novels because of gratuitous and badly-written sex scenes, than that thankfully is not a problem here.)