Which is better for live streaming, RTMP vs HLS vs WebRTC?

It depends according to your streaming project.

If latency is not very critical and 8–10 seconds latency is acceptable, HLS will be the best in terms of scalability and quality.
If latency is critical, the solution may be WebRTC(0.5 – 1.0 sec) or RTMP(2–3 secs)
WebRTC is supported by browser natively and there are some WebRTC media servers. Nevertheless, it is a new technology.
RTMP is not played natively on browsers. It requires flash plugin to play and flash support isn’t available on mobile browsers and its desktop support will end. Although, RTMP is old and mature technology.
I am a developer for Ant Media Server which is open source and free media server supports WebRTC , RTMP and HLS streaming solutions. There are also other solutions on the market like rtmp Wowza, Red5pro, Kurento, etc.