Personally I always loved "patching" style environments like Pure Data for fun and experimentation only reaching for the likes of Csound or Supercollider when I wanted extensibility or portability.

ChucK I've had the least experience with of those more mature toolkits (I didn't even know it was still actively maintained or developed), however I'd thoroughly recommend Supercollider due to the emphasis on live coding, mature community and integration with many languages.

Unfortunately I cannot speak to the qualities of Sonic Pi, this being the first I've heard of it, although I must say it looks great!