You forgot The Pitt. Bah, who cares? Nobody likes The Pitt... (twenty bucks says the next person who posts likes The Pitt.)
As for me, I prefer Operation:Anchorage. Three words: Indestructible. Gauss. Rifle. I know, I know, that's cheating, but even the regular Gauss Rifle is one of the most powerful and efficient damage-per-shot weapons over all of the expansions. The only non-Big Gun weapons that do more damage per shot are the Tesla Cannon and Alien Blaster, but the former takes longer to reload, has no scope, and is much more dangerous to use at close range, and the latter has very limited ammunition, even with the expansions. Even with its single-shot magazine, the Gauss Rifle is pretty useful at point-blank due to its knockdown effect. If you think about it from a different angle, it's almost as though the Gauss Rifle never has to be reloaded at all; it just has a very low rate of fire.
Another perk of the weapon is that its scope is much easier to use than that of the regular Sniper Rifle. In the Gauss Rifle's case, there's a dot that designates the exact center of the scope, and it's bright red, so it doesn't blend in with the surroundings like the standard sniper scope tends to do. The regular Sniper's scope leaves a lot of room for error by being dull and imprecise. Even if you can actually see the scope's cross hairs while aiming, you basically have to guess where the middle is.
Finally, concerning the expansion as a whole, beyond just that weapon, finding a mutilated Gary corpse in the Outcast Outpost was priceless...
