Really depends on the roleplay or the scene. But...

Medieval fantasy typically demands medieval music or video game soundtracks. Weirder, darker, or more sci-fi related medieval settings call for the seriously amazing [url=https://www.youtube.com/watch?v=XhG6Qaij7VE&list=PLwNcU6UOEFaxMVCHUYQqWRTXJaBFUs2Qp]Endless Legend Soundtrack.[/url] This game has a nice variety of tracks I can pick and choose, and none of them are intrusive. They let me think as I write, and they all sound beautiful. Other video games with good medieval soundtracks include [url=https://www.youtube.com/playlist?list=PLMbw15ySmbBu2ryOh0vtNjqMFDnJLSG7S]Battle Brothers[/url] and (for exploration stuff) [url=https://www.youtube.com/playlist?list=PL7n9Lk9WUhM--QO2z4VyhYFU-lBHIPlbq]Expeditions Conquistador.[/url]

(If you like weird fantasy music, you may also like some of Keith Zizza's work, like [url=https://www.youtube.com/playlist?list=PLJl31IQYN5EkMd003GHm4E4LRv57DpcJU]this playlist,[/url] or maybe [url=https://www.youtube.com/watch?v=VyGT-mfAD6k&list=PL37F122748AADD787]this one.[/url])

Civilization 5 has some amazing soundtracks, too, but you need to really know what you're looking for when you start making playlists out of anything that game has. Its soundtrack covers music meant to represent the ancient era to music meant to represent the modern era.

If you want to listen to some neat sci-fi tunes, [url=https://www.youtube.com/playlist?list=PL34B17023CC305F3D]Endless Space[/url] has some AMAZING tunes. One of my personal favorites - one that's really easy to relax to - is [url=https://www.youtube.com/watch?v=P5zdlEqut6Q&index=20&list=PL34B17023CC305F3D]this track[/url]. These are the same guys who made Endless Legend, so I guess it's not surprising that both games have great soundtracks. The Mass Effect series also has some good tracks.

Character-focused music is a common thing I use when I write. I.E. angry music for an angry character, or medieval Italian music for a medieval merchant / schemer. I've been preparing two playlists for a raptor superhero I made in an RP. Her battle music has stuff like [url=https://www.youtube.com/watch?v=4i3l_c6Nq9I]this sweet Killer Instinct track[/url] and also some select hip-hop / techno that fits fight scenes.

But the general rule of thumb is to find music that gets the message across and is non-intrusive. Sometimes you need to break that rule, but most times you need something that lets you think while you listen.