The answer, however irritating it may be, is the one most accurate in your rifle with a bullet suited to muleys. Of course, until you test a few loads, there's just no telling what that load is.
Some go-to bullets in .243cal are the 85-100gr Partitions, 90-95gr Ballistic Tip, 90gr Accubond, and 90gr E-Tip, as well as the Barnes 80gr TTSX, and the 80gr Hornady GMX, and the 85gr Hornady Interbond. If you can find the Speer Hot-Cor 105gr, it may also work well. There are others, I'm sure. Just about anything in the 85-105gr weight range, of medium game taking construction, will do fine if placed through the boiler room.
Powders to consider will be everything from H4895 and H4350 up to Accurate MagPro and IMR7828ssc. Anything in those burn rates will work well. Let us know what bullets you want to use (I'm no fan of sierra) and we may be able to better help on powders.