ODST has a special place in my heart for basically being Halo: Firefly Edition, but I can't even begin to explain how much fun I had playing 4-player campaign on Halo 3.
I'll never forget the time my friends and I were getting our asses handed to us on The Covenant (legendary) because one of us kept trying to ramp the warthog off the cliff to land on one of the Scarabs. We all just dismissed him as useless and laughed when we'd see a random warthog fall into the canyon. He spent about 20 minutes doing this. We finally decided for the 3 of us to just ride the hornet and have one drop on top of the Scarab to make it easier on us, but we kept getting hammered by the brute on top of it with the Fuel Rod. All of the sudden, we see a warthog glide through the sky and right on top of the Scarab, splattering the brute that was shooting at us. One of the best moments I've ever had in gaming.