Halo 3 was a great game. I want to play it so bad, I haven’t played it for 7 months because I don’t have a 360 anymore 
But man, I am just starting to really miss playing Halo 3.
I loved all those Pre-DLC maps especially 
Sandtrap
Construct
GUARDIAN!!!
LAST RESORT!!!
THE PIT!!!
High Ground
And Snowbound… Those were my absolute favorite Maps not only in Halo 3, but even my favorite multiplayer map from any FPS game out there! Bungie really hit the bullseye with those maps.
The Campaign was marvelous too! I hate it when people say Halo 3’s campaing sucked… sure I wish it was longer, but I still think it is a master piece!
Some honorable mentions for my favorite missions:
Tsavo Highway: Had some good LAN Co-Op Memories with that one 2 of us would be in the Warthog, the other 2 in Choppers.
Floodgate: Halo 3 was my first time ever playing a Halo campaign (before that, I would just play multiplayer with my friends at there house) and so, this mission was my first time seeing the flood, and let me say it was quite the experience.
The Covenant: This level was just fun to play. It had just a little bit of everything. It had grass one part, Snow the other part, and then forerunner here and there. It had Hornets, Scarabs, Chieftans, Flood, as well as a copious amount of on-foot combat. It really gives a taste of every form of Halo combat. Very fresh level, always fun to pla
Cortana: As you read above, Halo 3 was when I was introduced to the flood. So this mission was all slimy and wierd looking. I remember my first time playing this mission 2Player Co-Op with a good friend. We turned the volume up COMPLETELY (it was loud!) and turned off the lights. Good times. Plus, the flood is just a fun enemy to fight.
Well, those four were my favorite. I liked the other 5 Missions as well. I really do wish they made that “Guardian” Level thing. Would’ve been cool.
Anyhow, I am just really looking forward to when I can get enough money to buy an Xbox 360 again. I can’t wait to play the Campaign and then some of those favorite maps of mine
Long Live Halo 3!