His skills as a potion-maker cannot be denied, nor can his skill at combative magic. Snape is undoubtedly one of the better non-Auror wizards when it comes to knowledge and ability in curses, their counters, and the art of magical combat.
He is also one of, if not the single best, potion maker in the series.
So to answer your question: Neither, as a teacher. Snape is equally terrible at both posts for one undeniable reason: he's an awful teacher.
Simply knowing your subject is only part of the job of teaching, as Professor Binns demonstrates. Much more important is the ability to ably convey what you know to your students. You must be able to make them understand what they're doing and learning. You must be able to engage them. You must be able to adjust your materials based upon the needs of your students.
Snape tries to do this, but utterly fails. He is constantly sidelined by his personal desires for revenge upon the House of Gryffindor and Harry, son of James, in particular. He goes out of his way to favor his Slytherins and belittle the other houses (especially Gryffindor). Every student in the school knows it, and it can clearly be seen to impact his ability to teach a class.
How many students drop Potions like a bad habit as soon as they can? How many classes get sidelined by 10-minute long rants on how a particular student isn't exceptional? No good teacher would allow such things to happen with any frequency - they'd want students to love their subject and enjoy their classes. For Snape, it seems to happen at least weekly. He may have tried slightly harder in DADA, but his terrible habits couldn't be broken.
Whatever else Snape was - hero, traitor, potion maker, fighter, all of that and more - he was undeniably a horrible teacher.
To re-answer, based on your edits and clarifications:
Snape was, as I said, excellent at combat magic. He is, overall, a magical powerhouse. He's also talented. He is Occlumens enough to defeat Voldemort. He can fly without broom or thestral, yet his combat powers pale before Dumbledore or Voldemort. They were his clear superiors. Snape, possibly, could have beaten either of them if he'd had time to prepare and caught them unaware. They, however, could have defeated him without preparation or underhanded tricks. Ergo, he is clearly not the undisputed master of combat magic.
If you consider potion making, on the other hand, he is clearly the greatest brewer of potions in the series. He may not be the greatest in the world - we don't know for sure. What we do know, however, is that Voldemort was never seen to mix a potion (he instructed Wormtail in the creation of one, but may have been lying when he claimed to have invented it). Further, Voldemort never made extensive use of potions - possibly considering them beneath him or fit only for traps.
Dumbledore, however, was an alchemist of some note. He worked with Nicholas Flamel to discover uses of Dragon's Blood, after all. It's unclear if any of that skill translates to potion making, but it gives us a meter stick. Dumbledore was versatile and could do almost any magic that needed to be done. But when he needed a potion? He always turned to Snape. Clearly, Dumbledore saw Snape as the superior brewer of potions.
Therefore, by the only reasonable measure (his relative skill vs his peers) Snape was clearly better at brewing potions as opposed to combative magics (DADA).