Which ONE would it be?

It depends on what you are looking for, scenarios or skirmish. Personally, while I do like scenarios from time to time I favor skirmish for it's openness of different ways to tackle a given situation.

To that end, I would purchase Men of War: Assault Squad if I could only buy one.

I would go for the first/vanilla Men of War.

My reasons are:
- I plan to go for games that have become old recently, due to my middle-end laptop. The vanilla Men of War is one of them based on my standards.
- From what I've heard, the first Men of War focuses more on its single player campaign rather than its multiplayer. With GameSpy being forever gone, you can say that this second point is a bit invalid. But (I know I shouldn't use but to start a sentence, but I'll break that rule for now) if you had to choose between Men of War, Men of War: Assault Squad, and Men of War: Assault Squad 2, and were planning to play the single player campaign for most of the time, you would most likely choose the vanilla Men of War, since the single player campaigns of both the first and the second Assault Squad are basically the same. I excluded both Men of War: Red Tide and Men of War: Vietnam due to the fact that I won't be choosing any of them if given the freedom to do so.

I can tell you which one NOT to choose!

"Red Tide"! Absolutely CRAWLING with bugs. 100% GAME KILLING bugs at that. Red Tide is NOT finishable due to the "Dash to Kunikov" mission being uncompletable!