It depends on what you’re looking for. If you want Atmos in your room (ceiling
Speakers) then you need a receiver that can decode 11 different channels (7 surround speakers and 4 ceiling speakers). Those receivers unfortunately tend to be at least $1000. Guys that go high end don’t even buy a receiver, they go with a preamp processor and then have separate amps for their speakers. The Atmos stuff is really cool, but you can have a sweet system with just 7 surround speakers if you’re looking to save money.
As far as what makes your system sound the best, you’re better off investing the most money in your front 3 speakers and subwoofer. Those four items make the biggest difference (though some will say that room acoustic treatments are even more important but it depends on the scope of your project).