I have the other three. I can only buy one; please help me.

Personally I enjoyed Operation Anchorage more.

It really depends on what you want.
They're both mostly 'actiony' and linear, little exploration in either. If you like campy nonsense and want alien tech get mothership zeta, if you like serious stuff and want to shoot commie bastards and also get the super nice set of winterized power armor and chinese stealth suits, well get the anchorage.

I'd personally say get the Anchorage

They lowered the price to 5$! I can get both!

Glad to hear that!
Have fun, mate.

with Operation Anchorage there is a glitch you can use to get a bunch of weapons & combat armour/green jumpsuit that never degrade (and come with a cool per-war theme) along with high amounts of ammo for them