Is o'brien even technically an engineering chief?
O'Brien's official title aboard Deep Space Nine was "Chief of Operations," which is essentially the same as "chief engineer" of a starship, given a different title presumably because you can't have an engineer without engines, which a station doesn't have.
I'm not sure if it's ever officially established in dialogue or not, but I'm pretty sure that once the Defiant was introduced, O'Brien was Chief Engineer of that ship as well. He was certainly in charge of engineering. There wasn't anyone else in evidence who could be filling the post. And it would be odd of they called him "Chief of Operations" when literally every other Starfleet vessel we've ever seen has a post named "Chief Engineer."
Regarding the OP's question, I think my vote would go toward O'Brien, with LaForge being a close second. O'Brien seemed to have an immense amount of technical skill and was also one of the most well-developed and well-rounded characters in all of Trek.
And, I'm sorry, I love the TOS crew including Scotty. But if we are to go by actual on-screen evidence, Scotty was not the miracle worker he's made out to be. More often than not, when Scotty performed an engineering miracle, Spock was there making it possible. Whether it was doing a cold restart of the engines in "The Naked Time," cross circuiting to B in "Obsession," or fixing the engine imbalance in TMP, Spock was more often than not the one providing the real solution to the problem.