I'll give it a shot.
I think Art of War. It seems to feature on most "must have" lists of expansions for EU4 and I agree with those sentiments. The mechanics introduced for the Thirty Years War and Revolutionary Wars are really impactfull if you enjoy playing as a European state. Speaking for me, personally, that really helped flesh out the game.
Together for Victory is not a massive expansion in terms of content. The focus trees are probably nice if you really want to play as those nations (not me currently, though I own it), but the DLC suffers somewhat from the nations it touches upon being spread apart and somewhat peripheral to the action. In terms of general features you do get the spearhead battleplan, lend-lease and an autonomy system. I like the lend lease, but must say that I do not really use the other two. Your mileage may vary.
I think Man the Guns, with the rework of naval combat in partucular, will make me revisit Canada and Australia, and in turn make me get more out of TfV, so that is maybe worth keeping in mind. And in any case: lend-lease in particular does make the game feel more complete. If you really enjoy HoI4, getting it at some point is not a bad idea. But if you really enjoy EU4 as well? Art of War adds more to the game, in my opinion, though I am hardly an avid player of either.