TL;DR Summary for x265 Encode Settings

Set preset=slow. Then choose 1 following to override the default parameters. These are my recommended settings, feel free to tune them.

• 1 Setting to rule them all:
  • crf=19, limit-sao:bframes=8:psy-rd=1:aq-mode=3 (higher bitrate)
  • crf=20-23, bframes=8:psy-rd=1:aq-mode=3 (more compression)
• Flat, slow anime (slice of life, everything is well lit):
  • crf=19-22, bframes=8:psy-rd=1:aq-mode=3:aq-strength=0.8:deblock=1,1
• Some dark scene, some battle scene (shonen, historical, etc.):
  • crf=18-20 (motion + fancy & detailed FX), limit-sao:bframes=8:psy-rd=1.5:psy-rdoq=2:aq-mode=3
  • crf=19-22 (non-complex, motion only alternative), bframes=8:psy-rd=1:psy-rdoq=1:aq-mode=3:qcomp=0.8
• Movie-tier dark scene, complex grain/detail, and BDs with dynamic-grain injected debanding:
  • crf=16-18, no-sao:bframes=8:psy-rd=1.5:psy-rdoq=<2-5>:aq-mode=3:ref=6 (rdoq 2 to 5 depending on content)
• I have infinite storage, a supercomputer, and I want details:
  • preset=veryslow, crf=14, no-sao:no-strong-intra-smoothing:bframes=8:psy-rd=2:psy-rdoq=<1-5>:aq-mode=3:deblock=-1,-1:ref=6 (rdoq 1 to 5 depending on content)

Side note: If you want x265 to behave similarly to x264, use these: no-sao:no-strong-intra-smoothing:deblock=-1,-1. Your result video will be very similar to x264, including all its flaws (blocking behavior, etc.).