As a stand-up Peter Kay is very good, but his material has shown limited range, i.e. schooldays, his family. The challenge now is for him to move onto new subject matter.
Going back 15 years or so Ben Elton's observational stand-up comedy was brilliant and innovative but, after a while, you can't move it on. He was cutting edge in the 80s, by the 90s he was dated and I'm glad he concentrates on other media. I hope Peter Kay doesn't fall into this trap.
IMHO Eddie Izzard is the best - stage presence, timing, delivery, showmanship, glamour and a wide range of material - he can take the piss out of anything and can do his whole routine in French to boot.