Before I get to my list, I want to say that I broadly disregarded any Attraction or sticker cards from consideration
Giving up a lander or other expendable thing for this trigger is great, as is the ability to put the counter wherever you want
Enduring Tenacity is a strict upgrade for the classic Sanguine Bond, having more staying power and coming out for cheaper while providing a damage-based win-con for gaining life
He'd been trying to take advantage of Master of Cruelties, a card that had captured the imaginations of more than a few readers, and realized that Master of Cruelties doesn't actually need power to reduce an opponent to 1 life (although it's certainly worse against creatures if it doesn't have any), so he could use Heartless Summoning to play Master of Cruelties, since he was planning to make it unblockable with Rogue's Passage anyway
Hybrid mana cards can be cast using one of two colors, increasing deck-building flexibility
On his face side, he is Mono-white 3/3 for four that removes a tapped vehicle or creature when he attacks, granted the owner can re-play it, but if it doesnt have flash this is a mini temporary removal spell