My opinions on all of these are as follows:
Ajani– The most consistently powerful. A
watchwolf statline across two bodies for
is already good, and becoming a Planeswalker is gravy!
Tamiyo– Probably the card with the highest ceiling of the group. She makes artifact tokens. She's pretty easy to flip and has a game-winning ultimate. Her plus ability is even good at keeping her loyalty up! The big question is– are you flipping her by paying four mana and sacrificing clues or are you using a draw spell? I'm not sure this is a card I would pay 4U for, but it is insanely good off of a
Brainstorm!
Sorin– Actually my favorite design of the bunch? Maybe I'm just a sucker for
Syndic of Tithes and blood tokens, but something about this design just feels super elegant. I know Life Gain is falling out of fashion in the Cube world, but I think the fact that extort does damage might just get our boy across the finish line!
Ral– I was definitely expecting Dack but that's ok! This card feels like a shoo-in for every [[450]] [[Powered]] Cube out there. For everyone else, I don't think he's actually that great unless you're also looking at a Storm theme. Ral is definitely a constructed card!
Grist–I'm surprised the bug girl with minimal established lore is getting a spark walker, but its cool to see her again after three years! I think grist is the hardest to transform of the bunch and has a fairly mediocre backside. Unless she's supporting some sort of Escape/Reanimator theme in green-black that we haven't seen in this set, I think she may be somewhat safe to write off. However, using
Unearth in conjunction with Grist is pretty cool, so maybe there's something here I'm missing!
Prices allowing, I would say that I will definitely Cube Ajani and Tamiyo, test Sorin, and ignore Ral and Grist.