FOTR first, then ROTK, then TTT, though I appreciated TTT more after the EE.
FOTR has the best focus on the three main things I love about the films: Gandalf, the Nazgul, and the Ring. TTT loses alot of that, and ROTK brings it back a fair bit, but not to quite the same textent. FOTR is the easiest to tell, in terms of narrative, given there aren't many groups to cut to, so no cutting problems (like TTT had, though, again, less so in the EE). Moria was also incredible, and I prefer Gandalf the Grey to White, so generally FOTR always gets my vote.
They're all great though, so this is just nitpicking
-D