In fact, by the end of my first day of playing, I had earned almost 200 of them just by playing through the solo battles and completing missions
The best ones are probably going to appear in white and green
You can easily remove a big threat someone else controls and play your own one, all in one turn
Arlinn, the Packs Hope is about as obvious as it comes as a powerful two-color planeswalker, but since Gruul is so well situated as a color combination right now, it gets an extra bump up my list
"Well you still have to pay the mana cost, and it's instant spells only, how bad can it be?" It means you can draw lots of cards with Brainstorm, blink Chun-Li to get her out of harm's way with an Ephemerate, or make your board untouchable with a Teferi's Protection every turn