As we start off here, I'll state that I have a couple bias that keep me from pushing a card to the top of the list.
If you have to banish a card to use it, I downgrade it a bit. If you think they are worth the banish, respond with it!
If there is a best card in AD1, its hard to identify. I'll guess at Blessing of Lamashtu. I keep it or it's Favor form on 1 or two toons in my parties all the way through AD6. 2D damage bonus can be savior, especially when it has to be a wisdom or dexterity roll.
Melee - Shock Glave +1. It's close, but magic with a reroll wins the day
Range - Flaming Returning Throwing Axe +1. Recharge to add fire is pretty decent
Divine - Improved Guidance. +3 to any check and very easy to recharge. I've always liked this.
Arcane - Fiery Bolt. Thank you for putting this in AD1. It is strong enough to have been in AD2
Light - not much separates the choices here. Any that reveals to stop a combat damage and buries to stop all damage is acceptable
Heavy - Elven Breastplate. This is competing against a rather small field
Shield - Orik's Shield. I keep running into range damage.
Orb of Fire. Your arcane users get an item that is very useful to help other party members
Incanter and Toad. Your Arcane and Divine toons get some help with these
Blessing of Lamashtu. See my intro above as best card in the deck
Respond with your favorites. I suggest that you comment if you specifically use the card for certain party types (small vs large, melee vs spells, etc)