A: Tier 5 makes the most sense if any of the following describe you:
you're planning to publish frequently (multiple videos per week),
you're running or planning to run multiple channels,
you want the highest credit allowance for scripts/thumbnails/Intel without needing to think about it,
or you want API access for custom workflows.

If you're newer to YouTube and starting with one channel, Tier 3 or Tier 4 is often the sweet spot. You get the full feature set including Intel, plenty of credits to publish consistently, and you can always stack additional codes later if your needs grow. Credits also refresh monthly, and addon credits are available in the app if you ever need more.

My honest recommendation: start with the tier that matches your current publishing pace, and upgrade later as your channel grows.