I'd put it's campaign after CE's and ODST's. Reach is like a mix of the two with it's exploration of an unknown world and sense or ugency mixed with its camaraderie story but it doesn't quite feel urgent enough in story as you know how it's going to end and it doesn't build characters enough for you to care for them.
It's still a thrill and has some of the best, if not the best set-pieces in the series to date. It is also very balanced and paced very well (at least, if you like a challenge). It's rather short though. I beat it on Legendary alone in 2 sittings, totalling about 11 hours. I think this is the direction Halo 2 should've taken. It reminded me a lot of the Halo 2 E3 demo back in 2003.
Overall I'd put it second in the series to Halo CE with ODST right behind it.
