Top tier, probably should always be first picked:
Ajani, Dack and Ashiok seem a bit oppressive to me and i quite often wonder are they good for my cube environment. Ajani will probably see the boot after the new Nahiri comes, i just don't like cards that generate non-games and Ajani surely can arrange that for you.
Dack stealing a signet or even Gilded Lotus is pretty brutal at times, but at least it doesn't lead to armageddon or some other non-game event most of the times.
Ashiok was taken out of my cube because i thought it just wins every game when you get it turn 3. Hard to kill, comes early, dunno.. i might try it again some time.
Karn and Ugin to me are just boring cards. You will windmill slam them first pick, doesn't matter what colors you are you will play them (or at least should probably). I like harder choices.
Not to mention resolving Karn or Ugin means game really often, i try to avoid one card combos... ...unless i make the power max again some time.
Good cards that are fine first picks, but doesn't win games on their own that easily:
Venser could also be top tier because it can be really powerful, but i put it here, because it needs a little more work than most good planeswalkers.
Ral Zarek is basic value, i don't think it ever over performs, it just gives me some mana and does a lightning bolt + gives me some life, nothing exciting but still good.
Kiora i like really much, i probably overestimate it, but the +1 is really handy against so many situations, when you aren't taking damage, drawing cards and ramping is sweet, Kiora is the power level i like in planeswalkers.
Narset same thing, the power level is just perfect for my taste. Not that scary, doesn't blow the game apart right away and the ultimate isn't just autolose for the opponent either, but still can be devastating.
I placed Narset this high because if you first pick this, you will probably force yourself to blue and will get decent value out of this planeswalker. I don't think it's hard to pull off a deck where Narset shines in cube.
Cards that are ok, but probably shouldn't be first picked:
Sorin i don't like this for first pick, because two colors for a clunky effect. If you aim for the ulti, you will probably have a board presence that wins you before the ulti even hits. If you make 1-2 2/2 fliers it wasn't THAT insane for you.. dunno, i think it's a fine card, but the cube (or the pack) has to lack sweet spells if i first pick this.
Tezzeret in my opinion loses to the blue tez, because a) two colors b) search ability doesn't guarantee you what you want. I like the card and it's in my cube, but i would rather hope it wheels than go for it right away.
Garruk/Nicol Bolas can win games, but going for a multicolor AND a 7/8drop right away that isn't a reanimator target seems like a weak first pick to me.
Vraska, Domri Rade, Xenagos, Ajani Mentor, Kiora all of these are just fine, i would pick so many one color cards over these cards.
All of the Sarkhan's are pretty weak, but not so weak that i wouldn't want to see them in a cube. Unbroken can really do work at times.
3-color sarkhan is really powerful, but the main problem is that it's 3-colors more than the effects
Sarkhan Vol i think it's a good card in cube, but it's just so vanillaish.. +1+1 haste or threaten isn't sexy, but it does win games !