All the Scarecrow love here is awesome. 4laugh
I haven't read too many of the comics, but I've seen a lot of the animated series. So...
In no order:
Scarecrow- He has a lot of interesting stories and plots because of his fear toxin. I also like seeing all the spins people put on his costume! Zombie Nazi Crane (in an elseworlds thing) was weird, Animated Series Crane was brilliant (both main ones), snarky vindictive Year One Crane was my favorite.
Harley Quinn- She's my hero! She's always fun and happy, and I like how devoted she is to the Joker. I don't like that she and the Joker have broken up (forever ago, but I still don't like it!) because they're my 'one true pairing'.
Joker- Natch! I like the order v. chaos theme he brings whenever he battles Batman. Plus, he's classy as hell and really, really interesting.
Mad Hatter - I haven't seen too many things with him in it, but I really liked his Animated Series bit because it's so easy to sympathize with him! Plus, 'Jervis Tetch' is really fun to say. Unfortunately, he seems to be a ***** in the comics. gonk
Poison Ivy - I like how she and Harley are buds, and I like how her powers are so loosely defined. I mean, I've seen her talk through plants, accelerate plant growth, make them shoot thorns- and then there's the hypnotizing kisses and poisonous blood! *o*