To my knowledge there are 5 Crisis stories that DC has made over the years. Some were turned into crisis later, I think (Zero Hour wasn't always Crisis in Time, was it?) and some are not Crisis-y (Identity Crisis). What are your favorites? What are your least favorites?
Crisis on Infinite Earths
Zero Hour
Identity Crisis
Infinite Crisis
Final Crisis
I've read them all except Zero Hour and I must say I find them all very enjoyable. They're all unique and so much fun for me. When I was getting into the DC universe, sometimes they were confusing to me but I would notice a character I wasn't familiar with, look them up online and learn about them until I could finally read them and understand who everyone was and what they did. These books may not be good jumping on points for new readers, but if you're curious you'll want to know who everyone is.
It's very difficult for me to pick a favorite because it would probably change day to day. If you ask me right now, I'd have to say Infinite Crisis. I thought it was an excellent sequel to the original Crisis on Infinite Earths and it introduced Jaime Reyes and Superboy Prime.