I'd have to say the Happy Mask Salesman. He's always been a really interesting character in my opinion, so this answer isn't particularly difficult for me. There's plenty of things about him that really make him stand out to me. From his personality to his aesthetics, the guy is really interesting.
I'll go ahead and start with his personality. The Happy Mask Salesman is a cool character because of how mysterious he is. There are countless theories of who he is - some even believing that he's the Goddess of Time! While I don't subscribe to this theory, it is an interesting idea. Him being shrouded in mystery is intriguing, but even more interesting is how he behaves. He is generally very calm, cool, and collected, but he has another side to him.
As we all know, he has a rather violent outburst once he finds out that Link has not yet recovered Majora's Mask. This is the beginning of when his character really gets interesting. We see this more emotional side of him, and this naturally makes us not trust him. And yet, everything else about his personality - his amicability, his seemingly good intentions, his I-scratch-your-back-you-scratch-mine attitude - seems to make him a trustworthy ally. So there is always that struggle that the player has: is he trustworthy?
Once you recover Majora's Mask, he offers a final quote. This is the single quote in the entire Zelda series that I really ever remember. Unlike some of the other often cited Zelda quotes, this isn't meant to be too deep. It's simply a declaration.
If you dissect this quote, however, you find a more interesting message behind it. He's not just saying that reunions are up to you, he's saying that every decision you make has a reaction, and it is up to you to face these consequences. It is a powerful message, in my opinion, as his words of taking responsibility for one's actions is a nice theme.
Yet, the quote itself is multi-faceted. Could he be referring to his parting with Majora's Mask? Perhaps Link was merely his puppet, who held the bait before Link, goading him onward. Or is he referring to the possibility of the parting between Link and the Happy Mask Salesman not being forever? It's very possible that he's implying that Link will see more of the Happy Mask Salesman one day. We don't know. But it seems to me like the Happy Mask Salesman was not completely satisfied. Perhaps there was still business for him that needed to be seen through to its end.
As far as design choices go in terms of aesthetics, the Happy Mask Salesman is still top notch. We can glean much from the designs that the developers of Majora's Mask gave him.
I'll first analyze his posture. He's hunched over from the weight of the backpack. Yet, he is still rubbing his hands together with a sly grin on his face. This implies a cunning attitude, despite this weight on his back taking its toll on him. Perhaps this weight is actual anxiety from Majora's Mask being out of his grasp. Despite this immense weight beating down on him, the cogs in his brain are still going. He knows exactly how he will recover it.
And then we come to the backpack. The backpack itself is also a symbol that helps give us insight on this mysterious character. As I mentioned in the last paragraph, this backpack - and the weight it carries - is the stress on the Happy Mask Salesman. This anxiety that he is without Majora's Mask is a heavy load for him.
And finally, we come to his most defining features in his design - his masks. These masks all symbolize the different sides of him. Just looking at the picture on the right, we see two emotions on the masks that are quite evident. The first is the look of horror on the gray mask. This mask is his emotions displayed when Link returns without Majora's Mask. The second mask, one of cunning, is below the mask that displays horror. This is the most prominent mask, which is why it is the most human-like of the masks - save the Mario mask Easter egg. This mask is his most worn one, so it is most similar to him in appearance.
So this makes you wonder: was his outburst genuine, or was it a cleverly crafted ploy? I believe that this outburst was, in fact, a work of acting in order to seem genuine. His emotions, or lack thereof, are the selling point to urge Link onward. By seeming distraught - by seeming human! - he tricks Link into continuing the journey to recover Majora's Mask.
His clothes give off an impression of wealth. Yet this also is an illusion that he has so cleverly crafted. In creating this illusion, he paints a picture of trustworthiness, of nobility, and of power. He uses these ruses to gain Link's trust, in an effort to retrieve Majora's Mask. Everything, from the clothes he wears to the masks he uses, are carefully chosen to achieve what it is that he desires most - Majora's Mask.
This cunning is what makes him my favorite character in design.
