I am assuming that you mean the 7mm Remington Magnum, as opposed to the other various 7mm Magnums. You mentioned the 168 grain Sierra MatchKing --- so I went to directly to the Sierra Reloading Manual, since this one has specific data for the 168 gr. Sierra MatchKing bullet.
The most recent Sierra Reloading Manual lists "Reloader 19" as the most accurate powder for the 7mm Remington Magnum and a 168 gr Sierra MatchKing bullet.
It lists the most accurate load as 63.1 gr of "Reloader 19" (or RE19), at a velocity of 2950 fps.
Their test rifle was a Savage model 116 with a 26" barrel, and 1 in 9" twist.
If for some reason, you were referring to a different 7mm, Sierra lists the following as the most accurate powders for the 168 gr Sierra MatchKing:
7mm WSM RE 22
7mm STW RE 25
7mm Weatherby Viht N165
7mm RUM no listing
Sierra Reloading Manual