This is all a mater of opinion and time of release. I agree CoD 4 was a great game. But its network absolutely sucks compared to MSG4, Resistance and Warhawk.
When it came out CoD4 was by far the best shooter, however, it won't be a top 5 shooter by the end of November. There are so many great shooters coming in November and The vast majority of them will be better than CoD4, most of them being exclusives. (Resistance2, Socom, Gears 2, Killzone etc, etc, etc.)

Multiplats usually lack some of the finer details. Ask any 360 or ps3 owner about which game has better graphics and 95% will pick an exclusive. There are usually less glitches in exclusives. CoD4 was horribly glitchy for the first few months and it still has issues with its network.

Nah exclusives are always better imo.