Firstly, I want to say that this is probably the first Final Fantasy where I liked every character I met. A huge amount of whether I like characters in modern games comes down to the voice acting. So for this to have so many well voiced characters, even the random NPCs... I'm really happy with the work that was put in.
I'm putting down some spoiler tags but honestly they're pretty mild spoilers imho, so click away unless you're only just starting the game and want to be kept in suspense for everything.
For me, Aranea and Ardyn win big time. Aranea's opening line (SPOILER)where she purrs out "Hello pretty boy..." won me over from the get-go. Her voice is insanely lovely to listen to. Ardyn , on the other hand, was at times difficult to listen to for all the right reasons. He really captured the villain voice, and the scene in the train where he's all 'woah, are you really doing this, what the crap dude' was just fantastic for his voice acting alone. He has this kind of... curve to his voice that really works as a mysterious enemy who keeps helping you and leaving you wondering why.
Cid, Cindy and Iris gets honourable mentions, and of the main gang of guys I found that I liked them all well enough. I was kind of put off by Ignis but he grew on me to the point that I loved waiting for that moment when he'd declare that he's COME UP WITH A NEW RECUPPEHHHHHH.