World at War was very good, just the maps where huge for the amount of people that played in it and that was off putting for me so I didnt get to play it a huge amount.
COD4 was the last good COD released but spelled the end with the ranking up system.
MW2 was worthless, nothing but hackers. I didnt buy because no servers.
BO was very good but lacked full control of server that had to be done with 3rd party tools (thank god for sCon!)
MW3 the servers and gametypes where good if you could get in but it had console problems and I dont know anybody who could join a server all the time. I think it was designed that way but thats just me
BO2 well the fact it went on sale so quickly tells it all. No server = No game. I didnt buy it because lack of servers.
Id be happy not buying another COD title again.