They're easily the most prestigious; going to Oxbridge will greatly increase your chances of employment if you get a first when compared to another universities student who got a first but I've never really understood it. What should really matter is the teaching, which apparently most universities suck at anyway so then it comes down to what facilities they can provide; Oxbridge will obviously have some of the best. Then it should be student satisfaction rates, I mean if students aren't happy going to the university I don't think I could see it as one of the best; LSE is probably the best example; one of the most prestigious but quite poor in satisfaction rates.
Prestige is really measured by how famous they are and how they are viewed regardless of whether they are good or not; Oxbridge are frequently shown in movies as being two of the best in the world and therefore they are. Entry requirements is also another things that determines prestige; the higher the ER, the more that people will want to get into it to prove they are smart. Finally, how much employers put value in it determines prestige.
In terms of 'best', what are they the best in? I don't think they have the highest SSR but they are easily up there; they will definitely have some of the best facilities; the teaching will be something that is highly subject as they can't be the best for everyone but they are likely high up in here, given the satisfaction rates, just a guess though. Given all this, Oxbridge are without a doubt, the most prestigious, but they might not be the best; but what do i know, I don't go there.¯\_(ツ)_/¯