Talking about sales: Halo1<Halo2<Halo3.... yeah ... you are right, halos best days behind it.
Again, Halo 4 is being made by someone other than Bungie the studio that created the game, so that means it will get lower review scores than the past games. And neither Halo Wars or Halo 3: Recon will come anywhere near Halo 3's sales, so yes I believe that is proof that Halo's best days are behind it. Furthermore the correct order is Halo: Combat Evolved > Halo 2 > Halo 3 game quality wise. Halo: Combat Evolved (solid story driven FPS, the best one ever that wasn't a PC port). Halo 2 (started off well but too much time playing as the Arbiter. The Grunts are too cliched. Story fails to take off after the point where it should). Halo 3 weakest story of the bunch. Graphics failed to really use the capabilities of the 360, so most people refer to them as just Halo HD). Game Rankings scores: Halo: CE 95.4, Halo 2 94.5, Halo 3 93.352. I predict Halo Wars review scores won't get out of the 80s (disbanding the team is evidence that Microsoft didn't have much faith in it) and that Recon won't be as highly regarded by the critics as the first three (as it is only going to be a 3 to 5 hour game that doesn't deal with Master Chief and Cortana).
My most anticipated games: Whatever Hideo Kojima is going to do next, Final Fantasy XIII, Final Fantasy Versus XIII, Gran Turismo 5, Alan Wake, Wii Sports Resort. Cave Story Wiiware.
http://www.youtube.com/watch?v=zqqLMgbtrB8