Yeah, GorillaMan, that’s an interesting distinction. I guess the opera is supposed to be about what all women do, not about what all men do. Also, why are the names of some operas usually translated and others aren’t? Cosi fan tutte, but not Die Zauberflöte; Cavalleria Rusticana, but not (typically)* Il barbiere di Siviglia*.
Anyway, there are a few ways of measuring most popular. GorillaMan has already mentioned Most Performed and Most CD sales. There’s also largest number of recorded performances. According to ArkivMusic, there are 67 recordings of La Traviata, making it number one.
And the largest number of productions scheduled in a season. For example, this site, SashaCastel, refers to a NY Times article that puts La Boheme in first place in North America in the 2002 season with 207 productions scheduled.
Another indicator would be: When an opera house is in financial difficulty and they need to sell as many tickets as possible, which opera will they schedule? My guess is that it would be La Boheme.