I’m sure plenty will disagree with me, but unless you have specific needs, I’d suggest spending more time sourcing your media rather than rely on transcoding. Most formats of popular stuff are available and Jellyfin will happily play it natively.
Also be aware that transcoding is VERY cpu intensive, unless you have a compatible gpu/transcoder. I run a ML110 with a 6-core Xeon (12 threads) and if Jellyfin needs to transcode something, it uses all of that and still stutters badly when seeking.
If you do need to transcode because you can’t source the media in a compatible way - you may want to use something like Tdarr to transcode it before you try to play it, so it’s ready when you are.