I love how popular it is to bash Oblivion. It's like people forget how amazing it was when it was first released. Play it for 650 hours, find every single fault in it, blow them out of proportion, and suddenly it's Morrowind's bastard step-child instead of the worthy sequel it actually was. I'm not trying to say Oblivion was flawless, but its flaws have been seriously blown out of proportion.
mini rant aside...
Going to go with Darkfall. Had such a great concept but the no-name developers shanked the hell out of it. Glitchy, unfinished, and unsuccessful. Really disappointing, it was one of the only times after Star Wars Galaxies died that I actually felt interested in an upcoming MMO.
Honorable mention goes to Sins of a Solar Empire. I don't give it my top choice, because the gameplay is actually done really well. Some great space-based RTS gameplay. But the lack of a campaign was really offputting for me, especially since it was released as a full price game. Hoping that Sins 2 comes out with a sweet campaign to back the already solid gameplay.