In a way.. Commander Shran was and a friend as well.. That's why they played so well off of each other. Not to mention Jeffrey Combs is such a great character actor.
Other than possibly Shran, Archer didn't have an arch enemy for the entire run of the show. Often a reoccurring villian would be introduced and then defeated within a few episodes, one such example would be the Klingon Duras, also notable might be the Xindi Reptillian leader Dolim. Silik might also be counted, although he helped Archer almost as often as he faught against him.