The best one is often claimed to be Carry On Up The Khybur . Looking at the films today they actually appear, by and large, to have improved with age. The only ones to avoid are the last few, IMHO, where the regulars looked increasingly stale and aged while society, and society's humour, had clearly moved on meantime, Carry on Emmanuelle being a notable embarrassment. There is an excellent docu-drama, based around Sid James' affair with Barbara Windsor, Gor, Blimey!(2000) which is worth seeking out. I read somewhere last year that there is thought to revive the series and format. I hope not as they were very much of their time.
i like them very much. Up the Khyber is probably my favourite, I also love Abroad, matron, Nurse, Spying, cabby, Constable, Don't lose Your Head, kenneth Williams is my favourite Carry on star, followed by Charles hawtrey and hattie jacques.