As brilliant and lauded as Heath Ledger's performance is, I'm glad to see this on YouTube, reminding us that Mark Hamill is likewise a genius when it comes to portraying the Clown Prince of Crime:
Around the time that Batman: Mask of the Phantasm came out, HBO did a "First Look" featurette on it that actually showed Hamill in the sound booth recording the voice, and it was amazing to see. Sadly, it wasn't on the DVD. I'd love to find that on YouTube!
Update 12/22: Thanks to John D, HBO First Look - Mask Of The Phantasm: Part 1 and Part 2.