I believe in the past line out success was a really good indicator, but that was for a period in the early 2000s and hasn't necessarily held long term. But realistically no single stat will ever be a perfect indicator, the game is much too complex for that. Any combination or complex metric that does correlate with winning isn't likely to be permanent either, as it will necessarily change the way teams play, to both take advantage and combat it.
Personally I just wish there were more public stats available, and the methodology for recording them was more open and well defined - what constitutes "post-contact meters"? When is a tackle classed as missed? How are multiple players involved in a tackle credited? etc. Frustratingly this information must exist somewhere, as they have to guide the people recording the stats, but none of the providers seem willing to publish it.
