The ACLU grades the presidential candidates in seven categories: "humane immigration policy," "closing Guantanamo Bay and indefinite detention," "gays and lesbians serving openly in the military," "ending torture," "ending a surveillance state," "freedom to marry for gay couples," and "reproductive choice." Gary Johnson finishes first over all, followed by Ron Paul, Barack Obama, and Jon Huntsman. Tied for last: Mitt Romney, Rick Santorum, and Michele Bachmann. A few overlooked issues that might have changed the rankings a bit, pushing Obama down relative to the others: religious freedom, freedom of speech, gun rights, assassination, property rights, federalism, drug policy.
Clarification: While all those issues would have lowered Obama's overall score, adding drug policy or assassination would not have changed his ranking, since Paul and Johnson are the only Republican candidates who are better in those areas.