(spoilers)
(1)Severus Snape
(1)Sirius Black
To me, a very one-sided match.
Sirius seems awesome at first; from the perspective of a young Harry he's a parental figure who is quite indulgent. But he's not exactly responsible, or competent, and makes consistently pretty poor decisions. When we see him in the past, he's a nasty bully.
Severus Snape, on the other hand, starting late book 4 becomes quite interesting. He used to be a death eater but reformed. He's going back to the death eaters as a spy. But what keeps him loyal? Then later, by book 6, he does things like give solid excuses to Bellatrix, and kill Dumbledore, making us wonder if he's just very deep undercover. And of course he actually is. It's a fun little back and forth, and there are more reveals and interesting surprises about Snape than there are about...any other character, besides maaaaybe Dumbledore.
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Cats land on their feet. Toast lands peanut butter side down. A cat with toast strapped to its back will hover above the ground in a state of quantum indecision