For Call of Duty- Modern Warfare 2 was the turning point for the series- everything was downhill afterwards. I don't think there's any servers still up.
Battlefield- Bad Company 2 was and still is my favourite. Despite 2 successive BF games coming out since BC2's release, there's still a large player base.
Tom Clancy's Rainbow Six: Vegas is one of the ultimates in tactical console-based first/3rd person shooters. Very fun albeit a little dated. Still a lot of players online.
On PC, Red Orchestra is the winner hands down for me- the original has mods for Europe '44-'45 (called Darkest Hour), North Africa '42-'45 (Mare Nostrum) and the original- Eastern Front '41-'44 (Ostfront). Darkest hour still has populated servers and update support is being revived after a few years of developer dormancy- some improvements and changes of game features and mechanics will be coming in the next few months or so.
And the best part about Red Orchestra? It's so old even my MacBook can run it quite quickly on the highest settings. It takes literally no time to load it up either.