I've been playing Halo: Reach with all my spare time for the past few days, ever since I got Xbox LIVE Gold again. Due to the lack of me playing, I've stayed the same rank the entire time: Major Grade 3. However, judging by the way that my team comes in first almost all the time in a Team Slayer* match, or how I kicked the butts of those ranked even higher than General (formerly the highest rank known to Halo players) in a Free-for-all Slayer match, I should be a much higher rank, such as a Colonel, or even Commander! In Reach, rank is more determined by how often you play, if you don't quit most of the time and play all your games to their end, than skill (though it does take some skill to complete the Daily and Weekly Challenges, which reward you with huge Credit* bonuses upon completing them).
*Note: for those that know nothing of Halo or its online Matchmaking multiplayer system; Slayer is a deathmatch-type game, where the object of the game is to get as many kills on the other team as possible within the allotted time, or until you reach the kill goal. Credits are your experience points, and you can spend them on items in the Armory without losing your current rank and experience point total.