I would personally choose Detective Conan.
One: It has over 600 episodes right now. It's still on going (at this moment right now there are 666 episodes).
Two: I believe the main plot is around half of it's completion or not even that much. Thus there's a lot more to come for me to watch.
Three: It's an anime I like a lot.

16 movies, 11 OVA, 5 Magic Files and 8 Magic Kaito specials. There's also some live action dramas. I don't believe those count.