Many of the titles posted were either well reviewed or popular, and often both. How are they underrated??
Looking at metacritic:
- Mirror's Edge has an 81 metascore/8.2 user score. I think that's a fair score for a game that had some good parts but a really short campaign and wonky combat sequences.
- Titan's Quest: Immortal Throne has an 80 metascore/9.0 user rating.
- Indigo Prophecy got great reviews with an 85 metascore/8.1 user rating and sold well enough to allow the aforementioned Heavy Rain to be developed using a very large budget. Heavy Rain received an 87 metascore.
- VtM: Bloodlines has an 80 metascore/8.9 user rating. It had a reputation for being buggy because it was buggy, and it is well thought of nonetheless because of the originality.
- Far Cry 2 has an 85 metascore/5.5 user rating, and is one of the only titles in this thread to be actually be viewed as mediocre by users. It still got great critical reviews, and the people recommending it usually include the sort of caveats you would expect in an average game, not a great game.
I'm not even going to bother looking up Asheron's Call (hugely popular at the time and very influential), Doom 3, Trine, Crysis, NOLF, Oblivion(??), Chronicles of Riddick (one of the best regarded FPS of all time), or MW2 (??).
If you showed me a list containing the games mentioned here and asked me to speculate about whether people thought they were underrated or overrated, I would have been more likely to pick overrated.