What is felt to be the best in wall speaker for a large den?

None sound good.Good luck though!!

https://www.paradigm.com/en/architectural-custom-install

Paradigm’s LCR’s are amazing. But all of their Elite in wall and in ceiling products have tremendous performance. The Pro’s are great as well. 100% made in their factory in Canada.
You would want some type of a small subwoofer to mate with any in-wall speaker to fill out the bottom end. I would suggest looking at Paradigm’s Millenia Sub. You can wall mount it or possibly put it underneath a piece of furniture to hide and comes in black or white and has Anthem Room Correction.

If you are doing the whole British thing, remember Monitor Audio too. :)

If you can find a pair, Jamo made some in wall speakers years ago based on the Jamo Concert 8.  I couldn’t even pull up a pic or the name of them but I had a pair about 10 years ago.

They use Seas Excel midbass and tweeter, have a very sturdy enclosure that you mount between two studs, and they sound fantastic.  

First ask here where they should go. Have her mark the wall with painters tape. Not a little dot either, the whole square. Since she is the one in charge of your life, make sure she knows she is in charge of your life. Have her make each one. Assuming there is more than one. Get down on your knees or whatever is your usual begging posture and ask. While down there might be a good time to ask when it will be okay for you to use the den. Will plastic furniture covers be required? How long can you be in the den? What hours? Are drinks allowed?

Have her make a list of the music you will be allowed to play, and the volume levels (if any) but remember its all up to her if she thinks you're out of line even to ask that's her prerogative she is after all the man.

Oops, I mean woman. See what you did? You got me confused there.

+1 for erik_squires post above.

option 1 ....their ”standard” in-wall custom choices

https://www.monitoraudio.com/en/custom-install-speaker-type/in-wall-speakers/

option 2

for the premium WAF acceptance ...their MONITOR AUDIO SOUNDFRAME speakers. These can be mounted either in-wall or on-wall.

https://www.monitoraudio.com/en/product-ranges/soundframe/
https://m.youtube.com/watch?v=PrIGQBA9Olo
https://m.youtube.com/watch?v=7g99mP5Ypq0

First thing first, have you looked at the depth behind the sheet rock? Take some measurements before you consider the options for in-wall speakers. I would highly recommend M&K in-wall speakers, if your walls can accommodate these.

https://www.mksoundus.com/collections/architectural-series

Triad Gold or better. They don't sound like wall speakers. I use them for home theater and am definitely not sorry. You will need a sub of course but an in floor or wall sub will round out the package nicely. It will not be a high end audiophile system but you will enjoy it.

My view of in wall speakers is the same of that of Mr. Richard Vandersteen.  It has been reported that he said that when Steinway makes an in wall concert grand piano he will make an in wall speaker.