What is the most liberal of the Protestant denominations?
This is a good question, if only because it's nearly impossible to answer! Protestant denominations are, like any other group of humans, incredibly diverse, so each and every one has both conservative and liberal elements to it. If I called Episcopalians the most liberal, for example, there would be some devout Episcopalians who would rightly take offense at my claim, just like if I called Baptists conservative, I'd probably offend some very liberal Baptists. Your question is also complicated by the fact that many denominations are really already multiple denominations. So the Presbyterian Church of the United States of America is different from the Presbyterian Church of America, and although the former is more "liberal" than the latter, they're all Presbyterians.
At risk of oversimplifying, then, conventional wisdom does say that the Episcopal Church and the United Church of Christ are probably the most liberal, Disciples of Christ and Baptists the most conservative. Bear in mind, though, that in official theological positions all the churches of Protestantism are remarkably similar.
It's in how those beliefs about God and Jesus translate into political opinions concerning issues like gay marriage and abortion that differ. For example, Episcopalians and Methodists are actually quite closely related historically and theologically, but on the issue of homosexuality, they're very different according to this study: http://pewresearch.org/pubs/1159/homosexuality-protestant-view