I use an M1 Mac mini. It might be a little overkill, but I use the mini as a server for many things other than jellyfin too. encodes are lightning quick with no fan noise or heat and very low power consumption.

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.