Just starting out?

Start with StreamBeats (free, no attribution, YouTube-safe). Once you're streaming consistently and want more variety, upgrade to a paid service.

Twitch-only streamer?

Monstercat Gold at $7.49/mo is a solid pick if you love electronic music. Skip it if you post to YouTube, as their YouTube licensing is separate and complicated.

YouTube VODs matter to you?

Use Pretzel Rocks or Epidemic Sound. Both cover live streaming and YouTube under one subscription. Pretzel has a desktop app; Epidemic has the larger catalog.

Need a wide genre range?

Incompetech and AudionautiX offer the broadest variety — jazz, cinematic, blues, folk, and more — all free under Creative Commons. Attribution is required, but both make it simple.