Without a doubt, the philosophy of time. The philosophy of mind is a piece of cake by comparison. According to a story, possibly apocryphal, Peter van Inwagen was once asked why he didn't publish on time. "Too hard," was his reply. If it is too hard for van Inwagen, it is hard. According to Hugh McCann, "Few subjects in philosophy are as difficult, as exasperating even, as the subject of time, for few elements in our experience are so inherently enigmatic." (Creation and the Sovereignty of God, p. 68)
This puts me in mind of perhaps the stupidest commercial of the '80s: "Man invented time. Seiko perfected it." Stupid but stimulating: how many fallacies can you spot?