Best: Picking just one best thing from Empire Strikes Back is really, really hard. So much of Star Wars is encapsulated in this film. Yoda, Luke learning about Vader being his dad, Lando being cool, Han Solo also being cool, the Hoth battle, etc.
If I had to really just pick one it would be Yoda lifting Luke’s X-Wing out of the swamp after the young Jedi-in-training has given up. The way the music swells and we see how Yoda, a tiny weird toad-dude, is powerful through the Force is still magical to this day.
Worst: Nothing is bad in Empire Strikes Back. Let’s move on… Okay. If you really want something “bad” I’d say Leia kissing Luke, in hindsight, is weird.