Underrated:
World of WarCraft
For a game that's "really fucking good" to "best in the medium contender" in many counts, especially in areas that are often acknowledged to be high-priority factors (like responsiveness) or that are routinely panned but it somehow gets right (like implementation of open world), it rarely gets a good word put in its favor
Myth (1&2 with modern fan-patches)
Basically the best RTT games around that, unlike many games I'm most likely to praise, aren't deeply flawed in some ways but in fact pretty much flawless with every one of their component parts being stellar, from compelling narrative told from the point of view to a common soldier, to satisfying physics engine that sends limbs flying around
Dota 2
I don't like ASSFAGs, but it's also one of those games that does so many things so right (you could write a lengthy argument just about how the UI works or how Icefrog does balancing), is generally high production value, and yet people only talk about how they hate it.
Overrated:
Baldur's Gate 2 (but also other Infinity Engine RPGs)
I think AD&D is a terrible worst-of-all-worlds system that has negligible tactical depth (especially in more exploitable CRPG context), hinders rather than supports emergent storytelling ("roleplay"), and has the worst ratio of crunch to mechanical detail you actually get out of the system. A system-focused game (PS:T at least has the saving grace of avoiding this) with a bad system is irredeemable, although I think writing, characters, etc, also get too much credit compared to games of similar quality.
Classic Arcade games (let's say it counts as two so I won't have to edit/make second post)
A reference class thing: I reckon a game like Flappy Bird is generally viewed with disdain while e.g. Joust is viewed with respect. But aren't they fundamentally very similar? If you think about it, is it not the case that Angry Birds is "better" than Breakout? I prefer to judge games consistently on their own terms