actually, i do believe Raven's ticklish.
ITEM 1: We've heard Raven laugh involuntarily when Cyborg and Beast Boy were in her mind and they met her happy self.
ITEM 2: She may try to surpress her emotions, but it's not 24/7 like she'd have u think. We've seen her blush pleanty of times.
ITEM 3: She shares a friendly giggle, that's right, Raven GIGGLING, with Terra when Raven finally accepts her as part of the team (which of course doens't last long).
ITEM 4: When Cyborg pretended to join the H.A.Y.P. or whatever it's called, at the end Robin, Beast Boy, AND Raven insisted he be reinitiated by wearing a pink tutu riding a unicycle or sumthing like that. that's right, Raven in on a joke.
taking these into account, she's not 100% emotionless, which could mean she's hiding a deep ticklishness as well.