There was this game I got a long time ago for the Wii called the Munchables. Despite it being hugely under priced(it was $6 at Best Buy) I actually enjoyed it. Too bad I sold it, I want to play it again.
Obviously for me I'd say The Elder Scrolls because anything short of them being the most popular in the world is just underrated if you ask me.
Less obviously I'd say the Harry Potter games (PC, only the first 3 as I have only played those) they have, and continue to, given me many many hours of entertainment. Unlike most games (especially ones based off of movies/books) they were (except the first) free roam and had several side quests. Those games were and still are great IMO.
