Board Thread:Warhammer 40k General Discussion/@comment-117.192.179.237-20131224182556/@comment-3258762-20131225224532

The skill level of every Astartes varries from book to book. We have the ADB ones where one single-handedly fended off enough Word Bearers and a robot so as to create a barricade of bodies, and then we have the James Swallow ones, in which Nathaniel Garro managed to defeat one (Albeit after saying "even a Primarch would hesitate to face one of the Emperor's guardians")

But I'd say that, barring the Primarchs, the Custodes are the strongest fighters the Imperium has and, possibly, one of the strongest in the galaxy.