I've greatly enjoyed the various add-ons and would rate them in the following order. I'd be interested to know what other people think of them.

1.Point Lookout - nice, evocative scenario, good little quest with the chinese spy
2.Broken Steel - kinda like the main game, but entertaining
3.Operation Anchorage - good variety from the main game but not much looting
4.The Pitt - I still liked this but not as much as the above
5.Mothership Zeta - I suppose the change of scenery was nice but I found it a bit boring tbh