Favorite Halo game and why?

My first real Halo experience was playing Halo Reach with my little brother. My mom asked if I thought he'd like it as a present, and I said it seemed like it was a popular franchise. Course, I never had played it before but it had Co Op so we played together, I ended up getting fixated and then finished campaign too. XD I played it so much I had to buy it 3 times. It also has awesome enemy sangheili, and they became my favorite alien race. Ai and design, pretty solid. Modding Reach is fun too!