This is subjective, right? After all, there are already lists of underrated and overrated games out there on the net (like this and this), so I'd rather write about underrated and overrated games according to my subjective POV.
The Most Overrated Game: The Sims
To me, the most overrated game is The Sims. Sure it is the best-selling PC game in the history, it has spawned seven add-ons, it got 9.1 rating from Gamespot, and my friends who normally don't play games were addicted to it. In fact, even I found the game quite interesting at first. Building your house is quite fun, and it is actually quite intriguing to see your Sims interacting with their environment in (relatively) realistic way, like dancing to the radio or yelling with pain as they cut themselves preparing the meal (the latter is better, actually).
The problem is: once those niceties get old, the game quickly becomes a boring, mundane clickfests to keep your Sims happy. Your Sim is hungry? Click and feed him. Your Sim gets bored? Click and get him interacting with "friends". WASH, RINSE, REPEAT. The worst part of it is that, unlike the city in Sim City, your Sims are not autonomous, which means that you need to keep clicking to make them happy. This is probably the most mundane thing since Tamagotchi, and they dare to call it a game? Oh wait, there are expansion packs to keep things interesting. But frankly, if you need expansion packs to keep the game from boring, then there must be some serious problem with the game. And excuse me, even the expansion packs are LAME. Heck, if I want to get a hot date, I go out and really get laid instead of playing The Sims: Hot Date. Maybe the game will be more interesting if they launch The Sims: Interstate Fugitive, The Sims: Psychopath Serial Killer, or The Sims: Obsessive-Compulsive Exhibitionist. Now that's something we can never do in real life.
The Most Underrated Game: Jane's survey sims
You may be a little bit surprised if I say that the most underrated game is Jane's "survey sim" series; that is, Jane's US Navy Fighters, Jane's ATF, and all their respective expansion pack (or if you just want it all, go and purchase Jane's Fighters Anthology). After all those games sold pretty well and they were not trashed by reviewers either.
So why do I call them "underrated", then? Well, just go to a flight sim community and ask what they think to be the best flight sim: chances you'll get Flanker or IL-2 or LOMAC or whatever flight sim with manual thicker than phone book and requires goddamn months of training to master. That's not saying that they hate Jane's survey sims, though. In fact, some of them create mods to enhance the series (like FA Futures). However, you will inevitably bump into folks that complained about the lack of realism in Jane's ATF.
See, that's the problem. Game designers, reviewers, and flight simmers alike are putting more and more importance in realism than fun factor (at least CombatSim acknowledged the problem). No wonder the flight sim genre is dying, because it's not easy to sell a game when its target audience is shrinking. Well now, many hardcore flightsimmers may (and most likely to) disagree with my preferences on fun over realism, but to quote Scott "Zuma" Wolf, this, after all, PC GAME, not PC REAL LIFE, so excuse me if I play Falcon 4.0 with all realism turned off.
Is Jane's ATF realistic? Hell no. But it is fun? Hell yeah! It is a game that allows you to experience prolonged and exciting (albeit unrealistic) dogfight in a jet fighter --something that is getting rare today. It has myriads of aircrafts to choose, and if you're bored with the game's packaged missions, you can easily create yourself (with probably the easiest and the most user-friendly mission creator I've ever experienced). It so much reminds you of all the excitement of Chuck Yeager's Air Combat. Alas, it only receives lukewarm 7.7 score from Gamespot. And had it been released in today's world of IL-2 Sturmovik and Lock On: Modern Air Combat, I suspect the score would have been lower.
Fortunately, though, Jane's survey sims were released in older times, before realism became the single mantra, and when flight sims were still enjoyed by broader audience.