Longs Peak.
Out of the 14ers summited so far, this one is by far my favorite. 6 miles of trees, switchbacks, waterfalls, and a very large boulder field, followed by 1.5 miles of white knuckle, narrow, edge of the cliff "trail". This is by far not the hardest 14er to climb, but is also definitely not the easiest, and I believe that is what made it so enjoyable for me. Once you pass through the Keyhole, everything changes. You have to make sure each step is sure-footed, each hand hold is solid, and account for what other climbers are doing around you. There's points you're going to question your sanity, but once you accomplish the climb (and descent), you'll come to realize it was absolutely incredible and totally worth those nerve-wracking moments.
Would I climb this mountain again? Definitely. But after a little time has passed to where I forget how steep some of those drops are.