Halo 2A, especially those gorgeous cutscenes and beautiful looking, enjoyable gameplay. Halo 2 has always been my favorite.
I find it incredible at how many people rave about the way H2A looks as I find it to look very similar to H4. Though H4 MM was great, the art style was trash. Way too cartoony.
As for my favorite MCC version, I would have to say H2 classic. Being able to play the game online again after having it shut down for almost 8 years at this point is amazing. I think Microsoft made a terrible decision in cutting the servers. Sure, the original xbox live didn't have many people playing and it did take take a small amount of money to keep the servers running but we are loyal customers. We are fans of Halo. What I find to be really sad is that there are a noticeable amount of people who have moved to Playstation and Microsoft refuses to be loyal to those who are loyal to them. Watch them cut off live support for 360 games and watch me probably sell my Xbox.
Yes, I'm still butt hurt over the H2 servers.