I voted for Cesar Romero before I saw Mark Hamil, but the latter will always be the voice of The Joker to me. His interpretation is the most pristine to me, but I think its unfair to compare a cartoon to a movie or tv show. For animation you need someone who's talented and can voice their role but for live action you have to account looks, physical condition, and other things. I'm not trying to say voice acting isn't a medium that requires lots of skill nor am I denying there are truly talented people in the industry (Hamil is the best VA to me, period), but you could have a woman voice a male character and vice versa. They could have the most perfect voice that truly matched the character but everything else is handled by the animation studios and artists. And yes I know voice acting is more than just talking for the character, their lips have to synch with the character's mouths.
I'm saying all of this because to me Romero and Hamil are both the best, equally. Jack Nicolson is a close second. I respected Ledger's performance, it was very well done and his take on the Joker was very original, but due to personal bias I will always see him as nothing more than Heath Ledger just acting really well while wearing white face paint and looking disheveled. To me, Romero and Hamil ARE the Joker.
And yes I fully admit to being completely biased to the goofy old Adam West Batman tv series, its always been my favorite. His crazy laugh is just infectious to me and I can't help but laugh as well when I hear him.