Snape is my favourite character. The book is the exploration of three people's choices: Harry's, Voldemort's, and Snape's. Snape, by far, has the deepest and most human story. Harry and Voldemort are the extremes of good and evil, but Snape is the reminder that no one is all good or all evil. His journey was incredibly well written. I've always loved the Harry Potter books, but The Prince's Tale, I think, takes Harry Potter out of a children's fantasy book and into the realm of literature.
Would I want to be friends with Snape? Maybe not (unless I knew him as a child), but I cared about him deeply in the books, and as a character he is fantastic.
I also love Luna Lovegood, The name you forbade in this thread (The-elf-who-shall-not-be-named-in-this-thread), Hermione, Lupin, and Dumbledore (The utilitarian).
I would probably enjoy being around Tonks, but I don't like how her story arc worked out...