Holy sh*t, that Master Chief made my day!
Personally, I've mostly dressed up as characters from the Japanese culture. Himura Kenshin (from Rurouni Kenshin), Rex Raptor (from Yu-Gi-Oh!), Uchiha Itachi (from Naruto), Reno (Final Fantasy VII) and Roxas (Kingdom Hearts 2) are the ones I've dressed up as, next to random zombies and vampires on Halloween over the years. Currently working on two different versions of Boba Fett (ROTJ SE and a She-Fett), Atem (from Yu-Gi-Oh!) and always looking around for parts for a future Call of Duty costume (in the lines of MacTavish and Ghost).