I am far from being the biggest Batman fan in the world and so that makes me far from being the biggest Joker fan. But there's little doubt that both the Batman and the Joker do well when they are mixed together.
I don't know when I read my first Joker story, it might've been in Jules Feiffer's The Great Comic Book Heroes perhaps which reprints The Joker story from Batman #1. Certainly that story is a chilling debut for a really murderous and cold-blooded villain. Jerry Robinson gets the credit for creating the Joker and it's an amazing thing really.
By this time I'd seen the Joker on TV as portrayed by the quixotic Cesar Romero. That's my first imprint on the character really, and that version of the giggling lunatic with less direction than eccentricities is how I imagined the character early on.
But as DC began to take a more serious look at the the Batman and consequently his rogue's gallery under the guidance of Denny O'Neil and Neal Adams, the Joker became a bit more serious too and deadly to boot.
My all-time favorite image of the Joker is the cover to Batman #251 which shows the villain expanded to impossible size with Batman pasted to a card. His look became rather fixed, with an impossibly contorted face which wore a perpetual smile. (I know I'm not alone on this one.) His murderous tendencies overcame his quirkiness and he was once again a very very dangerous villain. The one artist who could render the Joker pretty well aside from Neal Adams was Irv Novick, whose angular style seemed well suited to the character.