Choose your successor!

Cora, since the human pathfinder would need to be a human, as has been said. Even if Vetra or which ever alien was offered the honor, would they even accept?

Also call me nuts, but if she hadn't gone bad, Sloane Kelly might have been a genuine candidate. High ranking in the Initiative, an Alliance officer, Shepard style "my way or highway" badass type. She would've handled the job in a renegade way (like she did the taking of Kadara) and quite possibly made enemies of the Angaran government, and maybe ruined humanity's chances in the long run... like fallen in for some simple trick of the Archon due to being a hotheaded idiot... But she definitely would have acquired a planet for humanity to colonize. Maybe if the Initiative had her back, she would have been more toned down too. Anyway, as she is during the game, absolutely not. (Haven't read that book and won't.)

Cora without a doubt. I don't like her very much, but she is the only person in my squad I consider useful. She is also the only person on the Tempest I talk to this time around. I call it my anger free play through.

Her voice doesn't do much for me. 

Logically it would be Cora, but after thinking about it, it would be a better story for a romanced Liam since he'd get access to Ryder's memories the same way Ryder's dad passed down his memories. It's kinda like Kasumi and her box in Mass Effect 2. If Liam wasn't romanced, then absolutely Cora, no question.