Both good choices! For me it's Dark Cavern. Seems like every time I fire up the Atari, even if I play something else too, I end up back at Dark Cavern. Other games that have been like this in the past for me include Midnight Magic, Pitfall! and Yars' Revenge.
@AAA177 I really enjoyed the arcade original, and this one captures the spirit extremely well for a 2600 port, it also has excellent sound design carried over from the arcade, and it helps in making it so addicting. Also it's just nice to have a 2600 game with more than a few screens lol.