Saying an effect like Stigmata or Kaos is an effect that any magician should own in my opinion is wrong. Not everyone is into that kind of freaky/bizarre magic. For resteraunt performers, you won't have a window nearby you constantly you can do it on.
Saying an effect that EVERYONE should own is well... none. It all depends on what kind of DVD you have.
Ahem... definitely Believe.
I do love this effect. One of my friends once told me on AIM, "Why don't you just do an Erdnase Color Change?" Something like Believe is awesome while an Erdnase Color Change is a small sleight that is fun to use in a variety of effects. Believe is just an effect that is a lot more magical. Sure you can make something like the Erdnase Color Change seem magical, but I like to use that during tricks and not as a trick on it's own. Believe is just... I don't know how to describe it really. It's really good and I love it. For 20 bucks you can't go wrong with this one. I bought the download.