Both PHIBES by a mile for me too.  I know Price himself preferred THEATRE but I've never been able to warm to it, although I give it a try every number of years.  I agree the "naturalistic" look of THEATRE (if intended to serve as a contrast to the tongue-in-cheek premise) doesn't serve it well; to my tastes the "campy" stuff (VP's lispy hairdresser schtick and Robert Morley and his poodles) is aiming low and off-putting compared to the more stylized outlandishness in PHIBES. And worst of all there's waaaaaaaay not enough of Diana Rigg for long stretches in THEATRE.