What is the hardest endgame to win?
Endgames with multiple queens both sides go beyond human understanding according to Jon Speelman...he studied them with the tablebase's and was clueless as to what was going on according to his endgame book 'Batsfords chess endings' which he co-wrote with Tisdall and Wade.
In practice, queen endgames (especially Q+P vs Q). Lots of checks and quickly you get completely exhausted calculating all the lines.
In theory, excluding more complicated situations (like R+B+N vs R+other color bishop), 2B vs N and some R+B vs R endgames like the Crosskill position. This is barely understandable.
What is the hardest endgame to win? Queen vs rook? 2 knights vs 1 pawn? 1 bishop and a knight? Let me know your answers.