I am wondering what endgame has the longest checkmate with optimal play? Right now, the longest I'm aware of is the two knights versus one pawn endgame, which in certain positions can take up to 115 moves for the knights to serve checkmate.
EDIT: I'm ignoring the 50-move rule for the sake of theory. FIDE actually changed that rule several times in the latter half of last century, allowing a 100- or 75-move rule in certain cases, but today they've restored it to strictly 50 moves
EDIT: I would define an endgame as each side having no more than 3 pieces, including kings. So this would be anywhere from 3 to 6 pieces total.