If you are only talking mainstream injection moulded kits, then the Monogram B-36 probably beats the B-52.
If you include limited-run resin, vacform or fibreglass, then currently it has to be the Amodel An-22 - see my build at :- http://www.flankerman.fsnet.co.uk/modl_an-22.html
Not yet released, but promised are vacform kits of the Antonov An-124 Condor and An-225 Cossack - and as these are, respectively, the worlds second and first largest a/c, then I guess they would take the prize...check out his website at :- http://www.heavykits.com /
Ken Duffey Flanker Freaks and Russian Aviation enthusiast. Flankers website at :- www.flankers.co.uk