Rook. The rook can cover every square on the board and can attack/defend from long distance. Where as the bishop can only cover half the squares and the knight is very much a short range operator.
edit: oops miss-read, I thought you meant out of the three.
alright, this position for instance
the knight is invaluable in a locked pawn structure, like the one above. The rook is struggling to find activity- if it was on the 7th or 8th rank, there's play on the e-file and perhaps the ability to get behind the pawns and pick off the weakest ones, but, as it is, it is a deadweight and will have to wriggle out of being trapped. Also, a lot of the black pawns are on red squares, which is something they are not supposed to be as white posesses a dark squared bishop. In this instance, bishop and a knight.
I wouldnt exactly want eithor one in this position...
but i would prefer bishop and knight in this one.
I'd rather have a rook. because their really powerful and if its just a knight and a king, it would be a draw! Same with the bishop!
Okay, that was a bad example, eventually white will trap the rook, here's a better one.
Personally, I'd prefer the bishop and knight over in any position because they can coordinate together and eventually overpower the rook. Josh waitzkin in "Chessmaster" proves why. He shows a game he played when he had a bishop and knight while his opponent had a rook and an extra pawn. Right from the beginning, it was easy to tell who would win,.
With some pawns on the board, would you rather have a bishop and knight, or a rook in the endgame?