Sonic Pi (IMO) is very quick to get started with because it's batteries included - you have everything needed in a single installer. Within the UI are tutorials, samples, synths, effects, a language reference, metering, 10 code buffers, and an OSC server built in - so once you config audio/MIDI devices you should be good to go. Sonic Pi is based on a subset of Ruby (though it's not clear which Ruby version, and what's left out) and Sam Aaron (dev) has indicated he's considering moving to another language. Though I haven't played around with Raspberry Pi, Sonic Pi integration is another benefit if you're into hardware.
I remember trying to install Tidal Cycles a few years back and having some difficulty - IIRC it's a library that requires other components/config. Same for Overtone [1] (Clojure frontend to SuperCollider), it took a bit of time to configure Leinengen at first.
Cabbage [2] interface to CSound would probably be closest to Sonic Pi, rather than straight CSound iteself.
Also worth investigating is Pyo [3] - a Python interface to DSP code written in C.
So many interesting (and free) choices - it really just comes down to your language preference. I make a lot of music in DAWs like Ableton Live & Apple Logic, but I have some fairly original, very specific ideas around exploring pitch, rhythm & harmony that a text-based language is better suited for.
