I actually think "Star Wars" is the best, but I can tell why people like ESB the most.
- Great lightsaber fight.
- Han be pimpin.
- Everything about Yoda rules.
- A darker tone overall (people love that dark stuff)
- Vader actually delivers on how cool he looks in "Star Wars"
- The Imperial March.
- Lando's betrayal is great character drama.
- The AT-AT fight is (IMHO) the best executed battle in the trilogy.