Aizen obviously. He's far more charismatic, menacing, intriguing and he has a certain aura about him. I am probably also biased based on their character and physical designs. Aizen just exudes elegance. He has many characteristics and physical features that I would strive to attain in real life. Above all though, it is his philosophy that mesmerizes me the most.
Yhwach on the other hand has the physical appearance of some homeless old guy. That's really off putting for me. I would've liked it more if Yhwach's appearance was that of his younger self. Secondly, he is driven by revenge and from the face of it, the tragedy he faced from being abandoned. It's a little generic. His dialogues are a far cry from those of Aizen's in terms of depth and intrigue. Having said that, I have come to like him more recently than I did initially. Last few chapters, his dialogues and actions have carried some weight. Let's hope the upcoming flashback endears him to me more. And yeah, that moustache is a big no no...