Hmm, perhaps I didn't phrase it well enough.
What I meant was, not the cards that gain the most from upgrading, but those which are used the most in common decks and which are difficult to win from False Gods - in other words, which are the best to spend 1500 electrum on.
For example, Improved Mutation has a large advantage over Mutation, yet it is used in very few decks.
The Cremations, although the advantage isn't so great, are still used in many decks, hence why they are a good upgrade choice.