Which one do you recommend?
If you want to focus on ancient Greece I would read “A War Like No Other” - about the Peloponnesian war between Athens and Sparta. Great stuff.
If you want a flavor of his work spanning ancient Greece, the US Civil War and WWII check out “Ripples of Battle”. A good short read at less than 300 pages. This book highlights what I like most about VDH; his ability to correspond ancient events to modern ones. Excellent book.
I think his best known work is “Carnage and Culture” - awesome book but somewhat long at 500 or 600 pages - can’t remember exactly.