I think the most fun I've ever had in the theatre - and, like Rrhain, I have a stage background, so I've had an awful lot of fun in theatres - was seeing
Peter Pan at London's National Theatre (the big stage, can't remember which one of the three that is), with Sir Ian McKellan as Captain Hook. Fun as hell.
Either that, or it was seeing
The Weir on opening night on the West End. Very cool, as well. Gave me the chills for the first time in years.