Best game of the franchise. period.
Here's what's good about Halo: Reach
1) Character Customization: In the Halo franchise, Bungie uses masked, silent (Halo 3: ODST) or near silent (Halo 1-3 and Reach) so that the players can feel that they are going through the adventure. This is no different in Halo: Reach but with an added bonus. When you first play the game, you have the option to customize your own Spartan, giving them different armor and choosing their gender. What's really awesome about this is the fact that the changes you make to your character's armor will show up in the game. So it will be your specific character playing the game.
2) Graphics: Reach has the best graphics of any Halo game. the character models and environments look very realistic.
3) Music: Martin O'Donnell has made some of the most fantastic scores in video game history. He continues this in Halo: Reach, and the music helps sets the dark tone that this game has.
4) Darker: This is by far the darkest game in the franchise. This is in part, for those who know Halo history, that Reach is a cataclysmic loss for UNSC. However, the game shows this also through it's imagery. The original trilogy's bright colored explosions and imagery is gone in this game. The scenery is more gritty and realistic.
5) Controls: The controls in Halo: Reach, like the rest of the franchise, is easy to use. The controls are near identical to that of the rest of the franchise minus a few changes. But even for veterans of the series, these changes will not cause any major problems after 5 minutes
6) Story: the campaign packs an excellent story to it. It's by far, the easiest story to understand of the series. But that doesn't mean it's any less great.
7) Intensity: Even on easy, the game's campaign is still pretty intense and decently hard. This helps get the player on the edge of their seat for the entire campaign
8) Armor abilities: Armor abilities, which are new to the Halo series. Although they aren't used much during the campaign, they make multiplayer more interesting and a little tougher though.
For those who don't know by now, this is going to be the last Halo game developed by Bungie. All future Halo titles (if there is any) will be developed by 343 studios. Bungie, who created Halo and made it into the phenomenon that it is, gave their it their all in their last Halo game. This is by far the best in the series.