Maybe not the best Halo, but certainly the best shooter so far this generation. From content to gameplay it's peerless.
So is Reach the best Halo game of all time? Yes and no. Yes, in that it has the most polished gameplay, the best online features, a STUNNING amount of content, a perfected firefight mode, and the best collection of weapons, sound effects, and level designs the series has ever seen. HOWEVER, reach still feels very familiar in many ways and thus does not have the mind shattering impact that the first Halo game had on so many fans.
The only way I could ever see a Halo game matching Combat Evolved would be a Halo game that is a launch title for a new generation of hardware that sets groundbreaking new levels of graphics, AI, physics, etc... the way Halo did for xbox back in the day.
This being said, i do think that Reach is easily the best shooter so far this generation in terms of epic co-op campaing, amazing multiplayer fun, a fantastic firefight and forge mode, excellent character costumization for all modes, and so so so much more. In terms of content AND gameplay Halo Reach makes almost all other console shooters look incomplete.
the sound effects are great, the graphics are finally great (some levels are a little "meh" though), the level designs are amazingly well crafted, and the story is epic and presented very well.
Bungie has left it's huge fanbase with a wonderful present, a Halo game that will be huge amount of fun to play for the rest of this generation. As long as they keep the best forge maps rotating through the playlists there is no reason why Reach shouldn't be the #1 online game for all bungie fans for a long time to come.
Halo 3 was great, but Halo Reach is easily the only game to come close to making me as happy as the first Halo did. Bravo bungie.