Thoughts on which dual-color Planeswalker is most powerful or versatile or useful? I’ve maxed the original 5 monocolor PWs; I’m looking for what next.
As of this writing we have three multicolor options:
Kiora MotD 
Sarkhan the Mad 
Ajani Vengeant
.
Depends on what your collection is, but Kiora is the best IMO. You can just run bounce, mana ramp, card draw (all of which is in common/uncommon), and build loyalty for a 32/32 reach trampler.
I chose Sarkhan because he currently costs much less than Kiora, but moreso for the reason you cited - my collection. I do not have a weak assortment of green/blue, but for red I’ve drawn particularly well. I have both Mythic Dragons, the Tyrant and Avaricious as well as the mythic Dragonmaster. I’m only missing the rare dragon, Akoum Hellkite. With those 3 mythics, it seemed an ideal setup for Sarkhan’s two dragon-based abilities.