Bitrate is king, and a lot of the stuff released is bitrate starved, for anime, sub 2k is likely trash. Most people don't post their metadata so know what an average filesize around respective bitrate ranges at your desired resolution is if you don't feel like downloading part of the first episode. Knowing the native res the show you desire was mastered in was important as well as whether it was made digitally or physically. Until half a decade or so ago, most everything was made digitally around or below 720p, so any 1080p release you'd get for such a show is just an upscale which was likely done no better than what you can do yourself with your own "live" upscaler like madvr.The exception is stuff made physically, which can theoretically be upscaled to whatever resolution if it was re-released/"remastered" properly. This depends on how the bd was done though so research on if the job was botched or not is needed to decide which resolution to grab. Use
https://github.com/Infiziert90/getnative . x264 vs x265 is miniscule. FLAC is bloat but is attached to most of the good releases, reencode it yourself.
Usually the best thing to look for is bd-raw encodes with little-to-no filtering then scrape whatever subs you deem the best off a release using animetosho.