I really hated opera until the time I actually went to see one. It was the opening of the season (not Glyndebourne exactly!) and I was invited in an official capacity. I went along for the champagne reception and hoped to slip away quietly at the first opportunity.
It was Così fan tutte performed in the open air, one of those low-budget productions performed by good singers past their sell-by date or promising young singers on the up, with sets that cost, oooh, tens of pounds! But by the end I was still sitting there as the applause finally died down, mesmerised.
It wasn't long before I was mainlining Wagner CDs, although I wouldn't recommend that for a beginner. If I had to take to their first opera somebody who claimed not to like opera I;d probably choose Tosca: it's comparatively short, it's got a good strong story and some magnificently hummable tunes including one in particular that tends to wriggle in the head for a week afterwards.