Best Variety of the series
Now i still think Halo 3 is the best in the series simply because it is the ending of the fight between the Covenant and Humanity, it also has the best ending in my book.
But Halo: Reach stands in at a close second, i thought ODST was the worst in the series and was only released to make more money (well everygame is, but they did not even try with ODST). i am glad Bungie is back on track and was able to finish their part in Halo in a big way.
The biggest improvement i would say is the graphics, in 2007 everyone thought that Halo 3 pushed the limits of the xbox360, but Halo Reach makes Halo 3 look as bad as Halo 2 looked when Halo 3 was released (makes sense?). i did notice framerate drops that were not present in any of the other halo games,but what can you expect? they continue to improve the gaming graphics on the same hardware.
Halo: Reach was not repetitive and was fun the whole way through (Halo: Combat Evolved is great but it does get boring at times).
The only bad thing i have got to say about Reach is that the Campaign is horribly short, even on Heroic Difficulty i was able to beat it in a little over 7 hours, although it was still longer than ODST but shorter than Halo 3.