I will have to go with Spore, since I've returned it and got a refund, the last time I did that was about a decade ago and it was for a console game which was really broken (N64 cartridge, somehow it refused to work), and it's the very first game ever I've returned because I actually got bored of it and saw no future for it in my book.
I'd have to give FarCry 2 the honorable mention, that one is also very close to be a "return it because it sucks" case, but I'll keep it, I'll just leave it uninstalled for the moment, the editor has potential as it is, it's not even the full SDK, and I do appreciate the multi-player enough so far, but right now it's on hold, I kept my saved game, I'll probably re-install it when a new patch comes out or when mods start to kick in.