I have Batgirl on the list to read still but I did enjoy the issues that were part of the various Batcrossovers in my current read for post NML Batman/Detective Comics/Gotham Knights
Gotham Central is my favorite but if that doesn’t count, then Catwoman (2002) followed by Nightwing.
For Post Crisis to Now, I’d have to say Duke Thomas is one of my favorite supporting characters right now, I love how he was built up during the New 52 and then got his own spot in the Bat Family sometime around Rebirth. But if this is Post Crisis to Flashpoint I’d have to go with Cassandra Cain.