The Hive.
Most of you probably haven't heard of it, but this map is the next Amplified. Plays so well. Look in the file share of GT: 'a Polish Korean' if you want to download it and try it for yourself. If you're a fan of fast-paced competitive gametypes, you will love it. Plays best with Gold Pro settings, settings very reminiscent of Halo 2. You can find those in the file share of GT: 'Gold Pro Customs'. These settings bring out the best in Reach while also eliminating much of the worst problems.
As for maps known to most of the community? Countdown. There is a huge problem in the map that is holding it back from its true potential, and that is the weapons layout. If that were to be changed, the map could be amazing. This was done in the MLG playlist, and it plays much better. Definitely a great map for KotH, Slayer, and CTF. Doesn't play too well with Bomb, Ball, or Territories, however, but those aren't known as mainstream gametypes. They are rarely played in default matchmaking. I guarantee you that this map will be known as being one of the best Reach, if not one of the best overall Halo maps. It's that good. The map could be the next Construct with a few fixes and possibly some modifications to the geometry.
[Edited on 11.17.2011 5:48 PM PST]