I really like the animated Batman universe, I prefer it to both the movies and the mainstream comics. Mask of the Phantasm was great, Return of the Joker was great (as long as you got the uncut version) and Batman-Superman was great.
The latter even got a nod in the mainstream comics, in an issue of 'Action Comics' where the Joker went to Metropolis specifically to mess with Lex Luthor. Harley and Mercy didn't get along any better in the comics...
Mercy VS Harley - Round 1
Mercy VS Harley - Round 2
Mercy VS Harley - Round 3
It's been about ten years since that comic came out, and I still think it's one of the best mainstream portrayals of Harley, those three pages are fifty times better than her actual series was.