When I started playing (what, two months ago?), I was trying to figure out which card to get to 40. I settled on heals myself, and I have zero regrets on that.
A level 40 heal card heals for 700 per pop!! It seems that most of the time that I battle against somebody else with heal cards, it comes down to who can out-heal the other. I get hit for about 1,400 per 9 seconds, and recover just fine. I return that 1,400 per 9 second to somebody with a lower-level heal card, and I'm going to come out winning that round.
So, if you are pondering the same thing, I highly suggest heals. Also, it is a silver card, so it is cheapest to get it to level 40.
My other 40's?
Bear Warrior: This was my 2nd 40. They start off pretty weak, but the more they get hit, the meaner they get. At level 40, they can take a lot of hits and keep on tickin'. Plus, with level 40 heals to back them up, they are an aggressive way to go. For the longest time, I ran with 3 level 40 bears and 3 level 40 heals, and I was taking down decks that had epics in them. Not a bad way to go, especially to start off.
Gargoyle: This was my 3rd 40. I did it for the extra 339 hit points, not for the extra 81 attack damage they pick up. These are my ultimate tank, and they are probably the unit I play the most. These are gold level cards, so it took a while to get this to 40, but it was worth it.
Meteor: 900+ damage, oh yes please!! Players quake when I throw a handful of these their direction.
The Hero: Another silver card, so cheap to level. I did this because when you level up this card, The Hero gains a lot in stats. A level 40 Hero gets an extra 219 damage and 1,039 health. That's 2,339 total!! However, I haven't been very impressed with this guy. He doesn't do anything special, and at 50 level defense, he is softer than I'd like him to be. Still, he can take a few hits. Often I'll combine him with a shield, but since it takes 3 heals to keep his health topped off, my deck gets pretty tight. 1 Hero, 3 Heals, and 2 Shield are effective against lower-level decks, but he still dropped to some of them.
Slashing Strike: I got the slashing strike to 40 to pair with the Hero. He can hit decently hard (the raw math is 1,531 damage), but I got much larger numbers with a level 20 Mia paired up with this, plus she comes out swinging a second sooner.
That's it! I did start a few other cards, and I tend to switch off between them here and there. These are some that have made it to at least level 20:
Princess Mia: She hits like an out of control truck, and that is without the slashing strike buff. It is rare for something to survive a hit from her, except the Specter. One of those getting healed will wipe out my Mia in no time. They also tend to make me cry. She is diamond, and is getting really pricey to upgrade. To move from level 27 to 28 they are asking for over 300 rubies. HOLY COW!! It the long run, it could be worth it. But it's going to take a lot of work! This is on my short list of cards to concentrate leveling up.
Blessing of Mother Earth: I got her to level 24 before realizing that I don't really play this card much. The 11 seconds to activate is pretty stiff, and a heal is more focused and is more likely to save my bacon. I usually keep her in reserve. At least it is a silver card, so it doesn't cost an arm and a leg in rubies.
Archangel's Courage: Well, I've got this up to 17, but each level up is so minimal in difference, I think there are better uses for my rubies. That said, my guys do hit 3.2% harder than an unbuffed one. Two or three of these and they are swinging harder. In the end, I also figured that I didn't see enough benefit from these to play them on a regular basis. It's a gold card, so expect some rubies.
Shield: I've got it up to 20, and will continue to work on this. I return an extra 19% damage to people who hit me. You hit me for 1,000? This will trigger right back and hit you for 1,190, and not even your armor will protect you. TAKE THAT!! This is on the short list to get moved to 40. It's gold, so the price in rubies is felt.
Sabotage: It's to level 18, but the price to upgrade is getting pretty steep so I might start here. I have seen 1 Sabotage take out as many as 4 spell cards (granted, I've only seen that once). At level 18, I have a 9.5% chance of destroying the card. If it triggers every 7 seconds, it is expected to activate 14 times. I'm surprised anytime this goes the distance without taking out a spell card. But since it is a diamond card, it gets really, really expensive to upgrade soon. I think upgrades past this point could be spent better elsewhere.
There are a few others, but the ones mentioned are the ones I like the most.
How about you others? What is/will be your first card to 40? (I still recommend heals) What other cards are you finding a lot of benefit at level 40 or higher levels?