Urahara would win, he was smart enough to invent the Hogyoku!
Szayel would win, he's just as creepy as Mayuri
Mayuri, he's crazy enough to blow up his own comrades
Buddy,u have just created the worst possible scenario thread lol
Seriously,even thinking about all the things this guys can come up with their twisted mind will blow my own and that of any other posters.There are cases where it's hard to confront 2 "simple" fighters....here we r talking about the sickest brains :-D
Ok,said that,both Szayel and Mayuri are obviously inferior powerwise.Brain-wise the matter is a little different.I think that naturally they r not equal,but for a simple reason:their areas of scientific competence should be really different...
_Szayel use his own body and living creatures to come up with new moves,power up,recovery,resurrection and many other thing.
_Mayuri can cover some area of Szayel's doing but likely not all of them;t the same time Mayuri has his freakynality in his poisons and all evil scientific things we can probably come-up with
_Urahara,differently from the other 2 he doesn't use his own body(or others living creatures) and he doesn't use poisons,so he shouldn't be skilled in these areas...at the same time he is the more skilled in inventing and using tools of different kind,being them for battle-use or other purpose..
In an overall scale Szayel should be the least intelligent but knowing from Kubo that the bodies will turn out to be very important for the future of the story,I would still wait to confirm this last point.
IMO we can say that Urahara is more the all-rounded-scientist but still I don't think that he can topple those 2 in their respective areas.
It would be interesting to know something about Aizen too,because he is another one with a great brain.Unfortunately we till don't know what are his areas of competence,but I guess that we will know it soon or later :-D