I like 5/5 for episode ratings, followed by a letter grade in the post if the user feels like clarifying. That's typically how I do it, anyway, with the scale essentially being 5 = A, 4 = B, 3 = C, 2 = D and 1 = F, with the +/-'s as the areas in-between (a 3.5 would be a B-, for instance). But for the poll itself, it's much cleaner to have 5 choices.
1/5 to 5/5 is the one I like the best. When you have too many numbers on a scale, the difference between the numbers diminishes too much. When you look at those scores out of 100, what really is the difference between an episode getting a 74 and a 75?
When you do 1/10 to 10/10, you're too tempted to do half-points and make the score out of 20. And again, what would be the difference between a 15/20 episode and a 16/20 episode? Not all that much.
5/5 is the best thing since sliced bread.