Thanks for the input guys, you made me consider some cards I might not have considered otherwise. Here's what I am at right now.
Azorius is between two cards for me. While
Grand Arbiter Augustin IV is cool, I think he's only my third favorite. For me it is between
Sky Hussar and
Isperia, the Inscrutable. Sky Hussar would suit my cube very well, but Isperia made me build an Azorius deck in middle school.
Dimir is easy. While
Szadek, Lord of Secrets will always be my bro (first legend I owned back when legendary was special), it was
Glimpse the Unthinkable that blew me away when I opened it in a booster 20 years ago. It also should have legs in my cube, either as a all-in selfmill enabler or as a gameplan with spell recursion.
Rakdos is easy too. I should've been cubing
Rakdos the Defiler already. Huge fliers are strong and often can survive to attack at least once. This version of Rakdos does not only have the coolest art, it also should be a great reanimator target, since it also let's people sac lands.
Gruul will probably get
Rumbling Slum. I remember reading about it in some magazine, but it doesn't seem as exciting today. I might reconsider this and don't change anything for gruul.
I like many cards in Selesnya, including all suggestions here. But how did I not think of
Watchwolf? I know it feels like a mediocre common to today's kids but I will always be happy to pick this and force Selesnya aggro.
Oh dear, of course,
Angel of Despair is one of the coolest magic cards ever and I need it here. Ghost Council is cool too but somehow feels less "legendary" than the Creature — Angel.
Golgari is difficult.
Putrefy bores me and
Gleancrawler is hybrid. I would probably play
Vulturous Zombie, it would be strong, but I don't like it more than any golgari card I run. I think I'll pass here, I am already running a bunch of monocolored dredge cards from this set obviously in the core.
Simic is even more difficult. I don't do the counters stuff, so
Coiling Oracle is the only card that would be an option. But I don't love it and I have pretty cool simic cards too. I might just pass on simic as well.
Izzet is a no-brainer. Or should I say a big-brainer?
Niv-Mizzet, the Firemind should actually be good in my cube and it's such an iconic card.
Boros is also difficult.
Firemane Angel is cool, but would it see play in the CCC? I might actually pick the super iconic youtube star
Lightning Helix, that works best with boros prowess too.