It's close between Dubya and Reagan. Dubya is worse because Reagan was at least able to contemplate raising taxes when necessary, and for all that his policy of "massive military spending to spook the Soviets" has put us in a bad situation budget-wise (and entailed a lot of morally dubious actions at the time), it did sort of have its intended effect. As for Nixon, anyone who signed the EPA into law automatically escapes the basement pretty much no matter what. The EPA is
that important.
Fun fact: I actually do accept that much of the anti-regulatory zeal of the Reagan presidency was actually warranted. But most of the rollbacks that actually made sense were actually the accomplishments of one Mr. James Carter, noted free-market reformer. For instance, freight railroads and beer were both saved by his deregulatory pen:
http://www.tnr.com/blog/jonathan-chait/76819/how-jimmy-carter-saved-beer#