Without a doubt, the best music to listen to while in a float tank is an instrumental and multi-layered track. When I say multi-layered, I am referencing the various instruments or types of sounds in a song. Everything from the drum line to the vocal track are considered layers. Playing a song with a complex and multi-layered structure helps to focus your mind on the music and can aid on taking you on a self journey. Now, I say this as a matter of personal opinion. The best experiences I have had in my tank have been listening to this type of music. Your best experiences may be had while listening to nature sounds or maybe electronica. The only real way to find what is best for you is through experimentation.
I would recommend starting with a type of music that is not heavy in one particular aspect. No heavy vocals, guitar, drums, and bass, etc. These can serve as distractions to keep you from focusing on yourself and your mind while in the tank.