That award without a doubt goes to iOS7 followed by iPhone OS 2. Both were WAY buggier out of the box than iOS8 IMO.Probably the buggiest iOS release yet, unfortunately.
Development on iOS7 was a nightmare for a while, the OS was severely unstable before 7.1 was released. iOS8 is adapting to a responsive display format, and that's without a doubt going to come with issues, but they're pretty small compared to bugs that iOS7 shipped with.