So, what's best and why, Own Opinions! There is no Right or Wrong answer
For me it was Call of duty MW2, I love COD4 and before that I started on COD5, Mw2 came at the perfect time when i had got to grips with all the games and was at a fair playing level with others, I pretty much got 10th legit, found a hack lobby at 10th rank 29.
I just preffered MW2 for some of the fast paced scenarios, Rust was a great map and I enjoyed sniping on that map a lot
All in all... MW2 just fits! An upgraded cod4, which was made to a decent standard (in my standards). Black ops 2 died quickly for me... What a waste of a few quid!