There were two dissertations both of them published in one peer reviewed journal, but one was recognized to be as the best and the other not, but with different titles, then what does that one to be the best dissertation?
That's a very hard question to answer. I don't think there's an "objective" answer to that, because the process is not objective. I've seen conference papers be rejected the same year they ended up winning best article awards. Like decisions made about hiring, or submissions to conferences, or other awards, much of this is based on who does the reviewing.
Some paper are placed in Web Of Science and other proof sources, may be it needs to be checked. Or in ProQuest Dissertations - this is also specialised source filled by best works. The new dissertation we can check "de visu" only, I suppose, or with expert's help and referee's review