I think BBvN would be tough too.
Hardest endgame to master?

NNNvB
Or a more likely difficult endgame is QP v Q
Thankfully, I'm pretty sure NNNvB has never happened in a master game (did it happen in a non master game?). As for QP V Q - aren't some of these drawn?
And about BBvN - isn't it drawn when the knight can't be taken quickly?
To me, QvR is extremely challenging, and most probably the hardest so called "basic" endgame to master. BN v K can be hard to some but I found out the technique is easy to learn.

This is what amazes me (from wikipedia):
A pawnless chess endgame is a chess endgame in which only a few pieces remain and none of them is a pawn. The basic checkmates are types of pawnless endgames. Endgames without pawns do not occur very often in practice except for the basic checkmates of king and queen versus king, king and rook versus king, and queen versus rook (Hooper 1970:4).

That's probaby not available to free members, right?
And yeah, I'm looking for a more interactive guide than a tablebase... something I could use as guidance in this complicated endgame.
As of now, I can't beat a computer in this.

I explore the winning positions on this thread:
http://www.chess.com/forum/view/endgames/queen-vs-rook-setup

That's probaby not available to free members, right?
And yeah, I'm looking for a more interactive guide than a tablebase... something I could use as guidance in this complicated endgame.
As of now, I can't beat a computer in this.
you can find several videos on YouTube, they cover some positions.

This site is good for practicing endgames, I also have Silmans complete endgae course which I am very slowly working through.
If you want a free site the following link in not to bad.
http://www.chessvideos.tv/endgame-training/endgame-simulations.php
Believe it or not I had trouble getting my head around the QvN endgame.

One can come up with all kinds of exotic examples, but I've seen enough detailed analysis of king & pawn endgames with multiple pawns to be skeptical that many players have mastered those.

The hardest one for me was always (and continues to be) N v B.
As far as exotic ones go, I once had Q v R B N (no pawns) against my computer (eek!).
I'd say QvR, but what about you?