Hunting muleys is probably the most optics-intensive hunting, in terms of how much ground you need to pick apart, and how well you need to do that, if you really want to maximize your success. But a lot depends on where and how you're hunting. Deeply forested vs high desert, backpacking in vs truck scouting, etc.
At a basic level, especially in heavily timbered country, you're probably good with just an 8x or 10x set of binos, along with a good spotter.
At a little more advanced level, especially in brushy and desert country, it's hard to beat a set of chest-binos at 8x or 10x, high-powered binos at 14-18x from the tripod, and a spotting scope. Choice of spotter is the most variable thing for me - the more foot-based the hunt, the lighter and smaller the spotter get. But I spend about 80% of my time behind the high-powered binos, picking apart the landscape in a very methodical manner. Spotter is most often used to double-check something I think I might be seeing, especially when on foot. Truck scouting can be pretty successful in some places with a high-powered, big spotter too. Chest binos get used for an initial glassing scan of an area to see if you can pick up anything moving, standing, out in the open, etc, and checking things out when you're on the move.