In no particular order:
Bruce Timm:
Apart from the TNBA design, his Joker is the very embodiment of the clown prince of crime we all love. Funny, evil and murderous, it was the classic look, a playful face with a giant demonic grin.
Alex Ross:
Love his black suit, and is drawn in such a way that red lips are not necessary. Like a lot of Ross' drawing, he is a midway point between the real and the unreal.
Graham Nolan:
This could possibly be my definitive Joker. Just look at him in Devil's Advocate, I think he might even possibly trump Bolland.
Tim Sale:
Love the exaggerated design as well as the more grounded Haunted Knight version. Love that jaw, he seemed quite deliberately like the Grinch, a cackling monster who enjoys taking away fun and lives.
Brian Bolland:
The Killing Joke. Nuff said really, his work is amazing, and every Joker cover art he does make me drool and despair that's as much as he does on the book.
Marshall Rogers:
His work especially in Strange Apparitions is to me the moment Joker was at his height. Still classy and funny, the drawing style really portrays tihs. Love the hair too.
Norm Breyfogle:
Haven't seen much of his, but just from the little I have he's already a favourite.
Dave McKean:
Arkham Asylum's version is suitably terrifying, giving off the perfect atmosphere for that particular story.
Whoever did the art for Gothma Central:
Oh man I didn't expect to like this, but I loved it. Joker as he should've looked in TDK imo, like a real person but still bleached.