Intel NUCs have the best balance between power, flexibility, and efficiency. That is pretty much end of story right now.

PIs are much more energy efficient at the cost of no SATA, much less transcoding power, less RAM, no PCIe expansion, etc...

A desktop PC has more power but at higher energy. It is still flexible, but you can't really get it below 30W idle.

A server PC has high energy consumption but with a ton of flexibility, capability, and power.