Alright, thinking on this some more....
Snape was a character who was absolutely amazing when the series was not complete. He was this mystery. After book 5 we were wondering "is he really on our side?" After book 6 we were wondering "is he really on their side?"
Now that the mystery is solved, however, I've...seen him described in these topics as "just a triple agent who does what he does for unrequited love", which...now that the series is complete, pretty much sums up his entire character. When the series was incomplete, it felt like there was a lot more to him; every little thing about him was scrutinized to try and figure out whose side he was really on. And...frankly, I was disappointed when I read book 7 that it felt like nothing new about him was really revealed. I mean, mostly, yes, because I had picked up on every single piece of foreshadowing, so I already knew he was in love with Lily, right down to small details like "he was the one Petunia overheard about the Dementors".
Dumbledore...ends up quite an interesting grey character. He writes to Grindlewald with stuff like "for the greater good", which is pretty much an "ends justify the means" argument. At first this can be dismissed as him being young, or being enamoured or influenced by Grindlewald. But once we realize how he manipulates and sacrifices everyone, certainly including Harry, this all starts taking a bit of a darker tone.
We also get to see a pretty significant character flaw--he feels shackled by Ariana, and wishes he could be out doing grandiose things. Which leads, of course, to Ariana's death. I'm not sure Snape really has a character flaw that's as believable and as interesting as this. Sure, Snape has the character flaw of "he's a total jerk, and thinks Harry is James", but it's hard to get fully into his head here, since Harry is actually very little like James if Snape's memories are to be believed. I also find it hard to be sympathetic for Snape being just an all-around jerk, because there...isn't really a reason for him to be. Not that there necessarily needs to be. But with Dumbledore we can get into his head, and see his motivation for why he would feel resentful of Ariana.
So...actually, something that's been bugging me: why does Snape want all of this kept a secret? Why is it so important that nobody ever finds out that he loved Lily, to the point that he tells Dumbledore that it must remain between them? I mean, other than the obvious "for plot reasons, this stuff was supposed to be a surprise, ergo Snape wanted to keep it a secret". Yeah, but what's his motivation for keeping it a secret?
Hmm...ok, yeah, Snape needs to win the character depth category by a non-trivial margin to get my vote, and right now he seems to be not actually winning.
(1)Severus Snape
(1)Albus Percival Wulfric Brian 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