Not sure if they really count as underrated, seeing as how they're pretty well-praised by most who played them, still I do feel the Sly Cooper series is relatively niche as far as platformers go, (possibly due to people looking at the games and associating them with furries) so I might as well talk about them.
The original trilogy in particular had great stories, characters and stealth/platforming gameplay. The first one, as much as it holds a special place in my heart is admittedly a game that was still trying to find its footing and was held back a bit by what was standard in 3D platformers at the time.
Second game was on a whole other level. It took the best parts of the original's gameplay and placed it into an open-world/mission based kind of structure, where Sly and crew do a bunch of small jobs that ultimately build up to a big heist at the end of each episode. Really made you feel you were playing as a group of professional thieves. Had quite a few twists and turns in it as well so it never felt predictable.
Third game suffered the same sort of problems a lot of "third games" in its genre suffered at the time. Shorter development cycle, loads of gimmicks and mini-games in lieu of actually good mission and level design, ect. Unlike Jak 3 and R&C: Up Your Arsenal however, I feel it did a satisfactory job overall and ended the series on a pretty high note story wise. (If you're willing to ignore plot conveniences and contrivances)
Also I always found the fact that the main character Sly, a thief, having flirtatious banter and romantic feelings towards someone trying to arrest him very quirky and endearing. Stuff like that really gives a game flavor, you know?
Less said about Thieves in Time the better.
