I defiantly picking up Demon Soul's seems pretty sweet, and hard. Darksiders is a meh for me I bought it on the 360 and wasnt too impressed. Will check out the others you mentioned. :]
Demon's Soul, Heavy Rain, Valkyria Chronicles, White Knight Chronicles 2, LBP 2, Ratchet and Clank, A Crack In Time, Disgaea 3: Absence of Justice, Uncharted Series, Sly Collection (I really enjoyed the 1st one),
3D Dot Game Heroes, Valkyria Chronicles and Folklore. I'll also throw out that inFAMOUS 1 & 2 and Resistance 1-3 are great games. More people need to play them.
If you don't have it on any other platform, Mass Effect 2. 5 bucks at Best Buy after coupon. Sengoku Basara Samurai Heroes is an awesome good time.
I'd play Disgaea 4 before I'd play Disgaea 3, FWIW. Even though 4 has less classes, it's easier to get into and play because of the improvements. Disgaea games rate almost universally as follows 1-2-4-3 with 2 and 4 being the ones that change position the most.
Resonance of Fate is niche but it's John Woo cinematic action RPG, definitely unique but it's hit or miss entirely.