Largest impact, sure. But architecturally it was a relatively simple formula - compromise a widely used package and sleep on it until it was pervasive enough to be a valuable hack. I disagree with this being the most sophisticated though. Unless I'm missing something about this hack, the Stuxnet[1] architecture, complexity, and long term planning feel far more sophisticated than the SolarWinds hack.
[1] https://en.wikipedia.org/wiki/Stuxnet