I don't recall Snape being said to be the most accomplished occlumens in the world. He's good enough to prevent Voldemort from reading his mind, but unless there's some indication otherwise that I've forgotten about, I think there's supposed to be a number of perfect occlumens, who are good enough that nobody is capable of performing leglimency on them.
I highly doubt that Snape is capable of performing leglimency on Voldemort. Not only does Voldemort have tremendous incentive to become a perfect occlumens, but if Snape could read his mind and not have his mind read in turn, he could probably have ended the war himself.
Not only does Voldemort have tremendous incentive to become a perfect occlumens
But training to become an occlumens seems to involve letting people perform legilimency on you. Voldemort seems to have an incentive to avoid that.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.