Just a short list of games (or series) that I think are underrated, or didn't get the love they deserved from the public.
Bloodrayne. <- Mindless schlocky fun that had you killing Nazis/Vampire cults as a Redheaded Dhampir Sexpot. Third game is coming out later this month, and it's a departure from the first two's full-3d style.
Primal. <- The only time Hudson Leick and Andreas Katsulas worked together, and I cannot heap enough praise upon the game for being a darker, thoughtful, and impressive game that really makes the PS2 work for its keep. Also has awesome Music from 16 volt.
Heroes of Might and Magic. (currently working on VI) <- Cult classic, but nobody gives it love because it's a turn-based fantasy Strategy when everyone wants Fast-pased FPS where you spend more time looking at a respawn counter than actually playing.
Sword of the Stars. <- Another TBS, this time in space, with the heretofore unheard-of act of randomizing parts of the Tech Tree. Voice Acting is hit-or-miss, the AI for some of the 6 races is schizophrenic to us, but actually makes sense based on the lore, and each race has its own way of traveling between planetary systems. Sequel is coming out later this year.
Metro 2033. <- A quote from one of the characters (who returns in the Upcoming Sequel; 'Last Light') "It appears that the devastation we brought upon ourselves was complete. Heaven, hell, and purgatory were atomised as well. So when a soul leaves the body, it has nowhere to go, and must remain here, in the metro. A harsh, but... Not undeserved atonement for our sins, wouldn't you agree?" Fallout Done RIGHT. The world is decaying; weapons are scavenged or built from spare parts, Military-grade rifle ammunition is used as your currency, all you have to protect yourself from the radioactive fallout seeping in from above is your geiger counter, a gasmask to prevent particles from getting into your lungs, and a handful of nearly worn-out filters, and, beyond the lights and squalor of the crowded stations, mutants and beasts lurk, waiting to attack from the shadows and devour you. Nobody I know has played it.
Titan Quest. <- Like diablo, only you're going after the Titans of greek mythology, and there's ragdoll physics so if you smack a weak enemy with a heavy weapon, they go flying hard and fast as their gear falls off of their corpse for you to scavenge. Never got much love because everyone thought it to be a ripoff.