Most forgiving high end speaker 10k-20k?

To answer your question more directly, I'd suggest Vandersteen models 2 or 3, both are under $4k. I've auditioned dozens of speakers and out of all of them they were by far the easiest to listen to, pleasant, smooth, no hard edges.

Onhwy61 made many good points.

Interesting question, and one I think is appropriate. What may sound spectacular in a demonstration may also wear on one's nerves over the long haul, for the reasons you mention. Onhwy raises good points also. I find Tyler Woodmere speakers extremely 'listenable' over a broad range of recording qualities. They are also very pleasing for long periods of listening. Even though they are "as good as what you feed into them," they have a way of remaining pleasing to the ears and emotions.

Well chosen system and proper set up go a long way toward getting the best out of ALL kinds of recordings. Onhwy61's advice is good as a starting point for building a system. The speaker, and its interaction with the room, is the biggest factor in determining how any system will sound, so one should start with finding the best speaker--even if that means stretching the budget there and waiting to upgrade anything else.

There are lots of candidates out there that are not overly analytical, lean and bright, so I hope some of my suggestions, and those of others, will work out for you. But, I hope that you keep in mind that we all have different systems and different ideas of what are more forgiving speakers and I have often been pleasantly surprised by hearing stuff that I did not like in several settings sound wonderful in another, so listen to as much as you can and keep and open mind.

In various settings I have liked the following:

Gradient Revolution --very flexible in terms of placement options, I've heard them sound good in small rooms and up against a wall.

Gershman GAP 828--warm and musical sounding with just about any kind of music.

Audionote speakers (various models)--plain looking (okay, perhaps ugly), but, they work well in corner locations and are never harsh or dry sounding. They work well with low powered tube gear which also helps in making bad recordings sound decent.

Magnepan 3.7--these require space around the speaker, particularly behind the speaker. In a good setup, these manage to be detailed and reasonably fast and dynamic without being harsh sounding. The tweeter on these speakers is amazingly good.

Vandersteen Quattro and 5A--I have not heard a Vandersteen that did not sound fundamentally musical and enjoyable with a wide range of recordings and accompanying equipment.

I have not heard they current lineup of JM Reynaud speakers, but in the past, I have found the whole lineup to be warm and musical.

Music lies in the midrange and achieving purity there goes a long way to making a speaker listenable and forgiving. When a speaker becomes less forgiving, it is usually due to problems in the treble, which can cause that grating, hard sound that annoys after a short time. For me, a speaker that is tubby in the bass, with one note flatulence is also fatiguing. I also prefer warmth over a clinical presentation. I want to feel the music, not disect it. So, the speakers I think do that best in your price range are:
Sonus Faber Cremona M or Elipsa
Aerial 7

I use the words warm and/or musical in place of more forgiving, but I think the Verity Audio Parsifal Ovations would fit the bill here. They certainly aren't the last word in transparency, but as you have noted, sometimes that can be a blessing. Combine the Verity's with a nice tube amp and I don't think any recording could sound bad.