That question really needs to be flipped (this is true of any "what's your favorite feature of [x]?" style question). It's far better to ask "what would you change about Xcode / developing for iOS[1]", because anyone can recite features of a product - it's another thing to be asked to critically think about what you would actually change to make your life as a developer easier.

[1] NB: "Crash less" doesn't count as an answer for Xcode, correct as it may be...