I know this is an open-ended question, but I'm hoping to generate discussion more than get hard and fast answers.
I play a variety of decks and like to switch up the strategies pretty often so I'm more interested in upping the cards the benefit the most from the upgrades, or that can be used in a wide variety of decks. I have a few hundred upgraded cards already and I'm sitting on enough electrum to up 10 more cards right now.
My instincts tell me that pillars and pendulums and novas are the best all-around to upgrade. So I'm leaning towards trying to up an equal number of pillars & pendulums for each element.
But some creatures (flesh recluse, horned toad, etc) get some pretty major increases/cost of casting. Others (puffer fish) change their skill.
Of course, weapons being upgraded might be more potent than upgrading creatures, but only to a point since there is only a single slot (or requiring flying weapons...which itself often needs upgrading to be most useful).
Maybe there are categories, like creature control (basilisk blood, freeze, lightning bolt and the rest of them) that are more useful upgraded than, say, creatures. Or shards, perhaps?
So...what (individual or classes of) cards benefit the most from upgrading?