Hardest endgame to master?

lol

(And I still say B+Kt is hard...)

I think he meant N vs. B with pawns.

Personally, I've found queen endgames in general to be the hardest because they tend to be the most demanding on your ability to accurately calculate, and theoretically won positions can be difficult to actually win in practice. At the very least, many of them require exorbitant patience as you watch your opponent check, check, check you, hoping that you'll eventually be able to take over the initiative yourself and make progress.

I've read that BB vs. N (no pawns) has been shown to be a forced win for the side with the two bishops (assuming they don't operate on the same color diagonals) via computer analysis. However, I think the win can be up to 278 moves long or something, so in practice, it's probably safe to say it's a draw.

QK vs RK is definitely much harder than NBK vs K

Q vs R or N & P probably.

Defending the worse side of R vs R+N for a draw seems hardest to me

Perhaps the hardest endgame to win in a game I've played is bishop + wrong-colored rook pawn vs rook pawn. In fact I offered a draw because I didn't see a way to win. (It was a theoretical mate in 55.)

How about Q vs QP?

I think I once saw a particular KNN vs KP that was a forced win with 55 moves.

What about K+5-6P on both sides. That's a heart breaker to lose if not hard.

Q v R

Defending the worse side of R vs. R + B is even harder. I wonder how many super GMs successfully defended theoretically drawn R vs. R + B endgames or reached move 50 without a clear mate in sight in theoretically lost R vs. R + B endgames. I definitely agree with all the other endgames mentioned here, but defending R vs. R + B is crazy.  

i think the most difficult endgame to play is queen vs queen and pawn

my top 10 pawnless endgames (not that accurate because i don't know much about endgames)

10. King and Queen vs. King
9. King and Rook vs. King
8. King and 2 Bishops vs. King
7. King and Queen vs. King and Bishop
6. King and Queen vs. King and Knight
5. King, Bishop and Knight vs. King
4. King and Queen vs. King, Bishop and Knight
3. King and Queen vs. King and Rook
2. King and 2 Bishops vs. King and Knight
1. King, Rook and Bishop vs. King and 2 Knights

Four knights vs Two knights

The really hard endgames are in the category of 2K + 3 or more other units. For instance K+2N vs K+P, K+2B vs K+N, K+Q vs K+R+P, K+2N vs K+R+B. Most of these endgames were not completely mined until the tablebases of the 21st century were constructed . Commonly they are too hard to master for a human which is why they are absent in endgame courses.

Bishop knight mate

I can't remember where I saw it but I can recall having seen that Q vs BN+rook pawn is winning for the queen side.

This link has a training dedicated to it, but it didn't say that this is a theoretical winning endgame: https://www.chessvideos.tv/endgame-training/queen-vs-bishop-knight-h-pawn-13.php

Seems hard enough to me.