Raspberry Pi is the most energy efficient and easy. Works perfectly if your clients can direct play the media. If transcoding is needed then you need a Celeron based NUC with QSV support (Intel Quick Sync). One of those can handle transcoding for a few clients unless we’re talking 4k source files then it’s like 1-2 clients max and no tone mapping. Next step up and we’re talking something that can’t really be called energy efficient.

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.

Honestly any mini PC made in the last decade should work. I would go on eBay and look at used machines. If you can go for something made in the last 5 years that has expandable storage.

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.

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The Intel NUCs should be good enough.