It doesn't have to be COD4 vs MW2. The two can co-exist as two of the best games of all-time.
COD4 is far more grounded and focused, with less content but it felt much more balanced.
MW2 is absolutely mental, but in the "perfect chaos" sort of way. Yes it was unbalanced and some things were broken, but fuck me was it fun regardless.
Both games had near perfect map rotations too. Maybe 3/4 maps between the two games I didn't utterly love.
If I had to rank my top 3 online FPS it would probably be:
1. Counter-Strike (1.5 through to GO, love them all in different ways)
2. Modern Warfare 2
3. COD4
As long as Activison don't mess them up (eg add lots of new elements which takes away from what the games originally were), I will play the fuck out of a remaster of COD4 or MW2.