sorry lesnar fans, the fact is that:
Boxing: Tyson kill lesnar in 10 seconds with ease. 100% Tyson
MMA: it could be both ways. but lesnar could win likely because he could be able to put Tyson down befor being killed by a punch... so lesnar more likely (75%)
Street fight: here it's hard.. there are too many factors. i know that it does seems similar to an MMA match more than a boxing match, but the truth is it's not. hit to the groin, open or close space, guns or object near, dirty tactics change all.
I think Tyson is favoured (80%) just for aggresion, ferocity, speed and the fact that i the Street it's more liekly start with punch than get down the opponent for a number of factors.. again lesnar would have the wrestling chance overpowered Tyson is almost impossible even for lesnar and Tyson is too Vicious (see what he did t holyfield).
Fedor or Velasquez would have an edge on Tyson for Street fight, and like 95% winning in mma. but i think that stret fight is not a benchmark (the most vocious could win as well as the first starter had an advantage, besides the space open or closed change a lot of thing.)
PS: i see all Tyson fight, see his book, and follow also MMA (every lesnar match).