The Harry Potter rifftrax have been some of my favorites!

I think the fact that the HP movies attempt to be serious, but don't always manage to pull it off adds to the riffing ability. (Similar to the B movies of old.)
That and the "Wizarding World" which is similar, but different from ours, tends to add some opportunities as well.
And of course the characters from HP are diverse enough, that it makes them fun to do voice overs for various "out of character" jokes.
I really would love to see the guys take on Goblet of Fire, Order of the Phoenix ect...
