I'm guessing it would be Washington or Jefferson or one of the early presidents. After all, they presided over the time of slave trade and made no effort to curtail it. Essentially they were importing a whole new class of extremely underpaid laborers, thus, sowing the seeds of class warfare.
Or maybe you could say Abraham Lincoln, since the slaves weren't citizens before he freed them. One could mark that as a critical step toward the creation of a new underclass.
Then again, if you're talking about income disparity gap, the dot-com bubble was a time when upper income increased incredibly fast, far outpacing the lower income levels, and this mostly occurred during the Clinton years.
Now there is a pretty good amount of evidence that the GW Bush favored wealthy people over poor and middle class, but he was far from the first president to do it and quite probably, not the one to do it the most. After all, the rise of the middle class has only been really taking off since the 20th century. It used to be that only landowners had any voice in government at all.
Don't get me wrong. The disparity between rich and poor is a very serious problem, potentialy a powderkeg for our society, but to lay all the blame at the feet of poor Dubya is stretching things quite a lot.