Bungie went out with a bang, for sure. Halo: Reach is a true gaming masterpiece, and a great exit for a great company...
The campaign was a lot darker than the other Halo games- knowing that everyone in Noble Team would die weighed down on you. The space battle level was a nice addition, though it could have used a little more.
Matchmaking was a true masterpiece. I spent countless days and hours working towards a certain rank, a certain armor piece. Nearly every day I would come home and get straight on Reach. Most of my time in MM was spent in SWAT. Gameplay was fair and balanced, with nice maps and a decent community.
Forge/custom games was one of my favorite parts too. Playing "Cops 'N Robbers" or "Dig to China" absolutely never got old. So many possibilities, with almost no limitations.