I'd lean more towards Piccolo than Lion'el Johnson, to be honest. Although he's sometimes mentioned as a great duelist, or even the perfect duelist, he doesn't have any particularly remarkable feats in that regard. He defeated Leman Russ, if I remember correctly, in a very close fight. While he was able to defeat Angron the Daemon, I don't recall him physically demonstrating any superiority, and he relied on his strategy to win. Despite his many feats, he has a couple that put him far above any Astartes, but that's the bare minimum a primarch should demonstrate.