Are you kidding? Midnight all the way. Its dark, atmospheric, and a world where the PCs might be able to really make a difference. The forces of good are down, but not completely out. Evil is dominant in pretty much every land, but some people still try to eke out an existance and fight against the corruption. Its a world where a few heroes might actually be able to accomplish something important, even if on a small scale. Midnight has the epic feel, without the epic magic and power inflation. FR, on the other hand, lacks the epic feel, but has the epic power inflation. Plus Midnight has the added benefit of not having a million badly written novels about it, so players cannot go crazy if you modify the world from what was stated in the novels. Finally, Midnight has no uber-NPC (Elminster and company, I'm looking at you) to ruin the world. If you haven't had to suffer through the Realms yet, save yourself some pain and go straight to Midnight. You won't regret it.