The attached poll is a list of the books for 5E that I do not own. I feel like bolstering by library, so I figure I would crowd source the order in which to get them. Note that I limited folks to 3 answers just to avoid folks voting for all of them. If it isn't on the list I either own it or am not the least bit interested.
By the way -- use whatever criteria you think and explain in the comments if you desire.
I chose Wildemount, because I prefer how they present the races and such in the book. In truth it could be a toss up between Wildemount and Mordekainen's.
If you are an Exandria fan it shouldn't even be a question. Get Wildemount. If you enjoy Exandria, that's what you should get point blank.
If you are interested in lore and a world, get Wildemount.
If you don't care about that stuff, hands down it should be the other choice. You should get Mordekainen's as it gives you more monsters and races. There's not much lore or anything else, but it does have a lot more races and monsters than any other book, including races that ARE monsters that you can play (overall...I don't really go for playing the Monsters type game though many do).
One caveat though, if you already have Volo's and Xanathar's...I wouldn't get Mordekainen. It's basically just a repeat with changed stats of those two books, but not as much lore or other material in it.
Tasha's is undeniably the biggest option of the books you list. It adds so many options to the game it's basically the Unearthed Arcana of 5e...it's just called something different.
I also chose the Essentials Kit because it's a great introduction overall, and for those learning the game, it's probably has the most content but in a simplified manner making it an easy book to utilize for those just starting out.
Edit: Got confused between Wildemount and Rising from the Last War (which is the one I would put over most of the other books except Tasha's...etc).