I wobbled back and forth between Lancelot and Percival in Excalibur. But I ended up going with Lancelot because he fails. He loses his paladinhood and experiences the fall from grace, then dies seeking redemption. Being a paladin isn't always easy. Certainly back in the day, it seemed like DMs were encouraged to jump on the slightest excuse to strip them of their power.
I remember rewatching the 2004 King Arthur and realizing how joyless and leaden it is, particularly Clive Owen's performance. It's got some good parts, but in the quest to make a "realistic" tale (something it still couldn't even manage), it strips out all the pageantry, wonder, and brightness of Mallory's tales.