I'd have to say Ledger is my favorite, but I get why so many didn't really care for his rendition of the character. In the beginning, Joker was this crazy and maniacal jokester who caused trouble just to get a good laugh, but as time and the early fans grew old, the makers started to turn Joker more darker, more Batman-esque, just in the evil way. I personally like that Joker more - beginning with the dreadlock look in the 90s anime - and you have to admit Ledger hit the nail on the head, even if you're a fan of the older joker. His character has evolved over time.
With that being said, I don't really care for any other Joker. IMO I don't count the voice actors, since you really don't have to get too "in character" for such. They have a talent, but that doesn't mean Kenny is anything relatively close to the Bob of Sponge or any other character his voice fits. West's Joker was eww, Jack was even worse (WAY better in The Shining), and the new Joker is the worst of the all. Seriously, just look at that new guy with the makeup. Main reason I never watched Gotham or Suicide Squad.
As for Quinn, the way the made her look also made me skip out on SS. I just love Quinn when she looks like the Jokette, or how I like to called it, Jester-like and all.