My situation may be unique, but valid - for those using iOS/OS X devices primarily and who usually listen to DRM'd/streamed music...
There are a few good ones.
Years ago I used Transcribe!. It was powerful and only ran on the desktop, but was available on Windows, OS X and Linux. Great interface, probably the best "sound" when slowing down and had great controls. Plain and simple, took an audio file, and modified the playback. Nice keyboard overlay for those tricky overtones or chords where you can't get it quite right.
Then I started looking at apps.
At first it was AudioStretch on iOS. But in all honesty I only used the Lite version (free). It worked well when I had DRM-free music on my phone or tablet, but since Apple Music came, and I drank the kool-aid; I've found many apps don't want to play the DRM'd streamed music anymore.
Except one I've found. It's called iLift and there's two versions of it. There's a standalone app that does all the speed/pitch loop changes you normally see in transcription applications; however it also has a version called iLift:Play that utilizes iOS's built-in music app to slow down up to 80%, loop, etc. (standard transcription techniques, but nothing advanced) and it can do it on DRM'd music since it's really just utilizing iTunes' advanced features.
If there's something more difficult I need to transcribe, I usually remove the DRM somehow and open it in a more advanced application.
In general, most applications out there (desktop or mobile) will do everything you need with a great sound quality and ease-of-use, especially if your music collection is more traditional with your own non-DRM'd songs available digitally.