There isn't an easy answer to that question. Any Roku device made in the last couple of years should be sufficient as basically they all run the same software, but it's usually a good idea to try your channel/app on a couple of different models from the low end (i.e., Express) to the higher end like the Ultra. The Express models may be the most common since they're the cheapest and anything that works on the lowest model should work on the others. I'd say just get whatever you can obtain easily and get started with that. I prefer the Ultra for it's wired Ethernet, but I wouldn't say it's absolutely necessary to have one.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.