A while back we had a competition arguing for this, with von Moltke being voted for in the end. I recently came across this character who looks like a contender, although frankly he is not memorable to me. He should have been... Viriatus of the Lusitani tribe
"Viriatus stands as the most successful leader in Iberia who ever opposed the Roman conquest. During the course of his campaigns he was only defeated in battle against the Romans once, and from a military standpoint can be said to have been one of the most successful generals to ever have opposed Rome's expansion anywhere in the world."
No mean feat.
https://en.wikipedia.org/wiki/Viriatus
Interesting read. Thanks for sharing Boggit