You're right, Snape is absolutely not likeable, and he's not even the character I like the most. He simply happens to be my favorite character, which is not the same thing.
The reason he's my favorite is certainly not because he's mean and nasty and unfair to Harry, and it isn't because of his looks (although the large aquiline nose is a selling point, I will grant). And I agree with you that the episode in the Shrieking Shack is calculated to present Snape at his worst (although there are some major mitigating factors there that are easily missed on first reading -- for instance, did you realize that Snape heard absolutely nothing of Sirius and Remus's explanation to the kids about Peter Pettigrew, and that when Snape came into the room he still had every good reason to believe that Sirius was a vicious mass murderer and that Remus was helping him? But I digress).
What sold me on Snape is the end of Goblet of Fire, where he rolls up his sleeve and shows Fudge the Dark Mark on his arm, obviously caring nothing for what Fudge or anyone else thinks of him so long as the truth is known. It was then that I suddenly developed my Grand Unified Snape Theory, which inspired me to write The Potions Master's Apprentice and, eventually, its sequels. (It was only supposed to be one short story, of course -- but that was true of Touching Indigo as well.) Anyway, the substance of my theory is pretty well expounded in the stories...
Even if I'm mostly wrong about Snape, though (I'm certain I'm not all wrong), he does have some definite things going for him: his absolute loyalty to Dumbledore, his determination to protect Harry in spite of his dislike, and fact that he repudiated his past as a Death Eater and turned spy against Voldermort "at great personal risk". Snape, to me, is fascinating because he's a redeemed sinner -- it's just a question of how far his redemption and internal transformation has actually progressed since he was saved from his old life by Dumbledore's grace. I happen to think that transformation has progressed further than JKR (or Snape) has so far allowed us to perceive, that's all.
I have more thoughts on Snape, particularly how he reminds me of a Dorothy Dunnett character, but they can wait for another time.