I wrote this as a post on a certain forum-that-shall-not-be-named. I'm saving it here in case it gets edited or deleted so I won't have to look up the Quidditch statistics again. Quidditch actually bores me to death, but there's no doubt that some truths come out about Snape due to the game in the first few books, so here goes:
The only people saying that Snape played favorites more than the other teachers were the Gryffindors. Actually, Hermione made higher grades than Draco, and that comes from Lucius Malfoy in Book 2. Harry and Neville both pass their Potions Owls, which tells me that Snape had really high standards and wasn't just teaching for the test, which was easier than the class.
I agree that Snape spoke in class with bitterness and sarcasm. I just don't see that he pampered Draco, since he was pampered by his own father on the School Board.
The "playing favorites" thing comes up with Harry mostly due to Quidditch instead of potions class, where Crabbe and Goyle are certainly not Snape's favorites.
So let's look at Quidditch for a moment.
First of all, Gryffindor wins every match against Slytherin in the books, thanks to Harry. So perhaps Snape had a good reason to book the practice field for his team, and since he is Head of Slytherin I can't imagine who else was responsible for doing it.
From Wikipedia:
1st Year/Slytherin: Harry catches the Snitch in his mouth; Gryffindor wins
2nd Year/Slytherin: Harry catches the Snitch; Gryffindor wins
3rd Year/Slytherin: Harry catches the Snitch; Gryffindor wins the match and the championship too
4th Year/Slytherin: No Quidditch
5th Year/Slytherin: Harry catches the Snitch; Gryffindor wins
6th Year/Slytherin: Harry catches the Snitch; Gryffindor wins
Yes, Snape was competitive about Quidditch with Minerva, but she played favorites just as much by giving Harry the Seeker position in the first year, an unprecedented move. And she bought Harry a Nimbus 2000 --- was that a fair move, considering that Harry could have bought it easily for himself?
Also, in Book One when Harry was in danger, Snape stepped in to save him during Quidditch, and even referreed one match, following Harry around like a hawk until he caught the snitch. He didn't follow Draco or any Slytherins around like that, as I recall.
The next time in the books that we see Snape on a broom is during the memories that Harry sees in Occlumency Lessons and Snape is on a bucking broom. He's not a natural on a broom, but more like Hermione. However, he is willing to fly around during Quidditch, not his natural element.
And we have this from Quirrelmort at the end of Book One:
Harry: "Snape was trying to save me?"
"Of course," said Quirrell coolly. "Why do you think he wanted to referee your next match? He was trying to make sure I didn't do it again. Funny, really... he needn't have bothered. I couldn't do anything with Dumbledore watching. All the other teachers thought Snape was trying to stop Gryffindor from winning, he did make himself unpopular... and what a waste of time, when after all that, I'm going to kill you tonight."
That is an interesting passage because it points out that even the other teachers thought Snape was giving the Slytherins an advantage (as did most readers at that time) but it wasn't true. Snape was there on the Quidditch Pitch only to save Harry's life and no other reason. Quirrell has no reason to lie to Harry - in fact he gives him the big reveal because he thinks he will be dead soon anyway. Voldemort and Harry had both defeated the last barrier to the stone as well - the Potion Logic Puzzle devised by Snape. That was not put there to protect Draco, or Slytherins of any kind. So he was also a "Man of Two Faces" just like Quirrelmort.
In CoS, Draco's father bought brooms for the whole Slytherin team. Now that might have given them an advantage ("might" because they actually lost) but it might also be seen as a balance to Minerva. Also, it ensured that any underprivileged child (as Snape had been) could afford a broom that year. So I don't see anything wrong with a Head of House treating his own students well. There's no proof that any other teachers did differently, and other Gryffindor teachers certainly treated Harry well - Hagrid, Dumbledore, and Lupin. Sirius buys Harry another broom in Goblet of Fire to replace the one he lost in Prisoner of Azkaban.
On the other hand, Harry is also Slughorn's favorite student - not Draco the Slytherin. It's possible that Draco might have been angry about that. So by the end of the series, Harry was actually much more favored than Draco in retrospect, and Snape doesn't even ask Dumbledore to spare Draco, but to let the boy go ahead and kill him. To me, he really cares more about Harry's fate than Draco's, but he still manages to save Draco by helping Narcissa. I don't see that as unbalanced, but balanced.
In the Seven Potters we see Snape again, and he isn't there for Draco that time either, and he saves a polyjuiced George who was disguised as Harry. So I think we can see a pattern there once again.