And which ones are being considered as popular nowadays in terms of number of live concerts
Once source to judge popularity in the current time is Operabase. Their database currently shows totals from over five seasons 2011/12 through 2015/16. They have information from 900+ opera companies around the world, but by no means is it complete. Here are the Mozart operas in the top 200 of their
most performed operas list.
Rank. Opera - Performances (Production runs)
Even if all performances of Die Zauberflote were combined together (as I suspect happens with most other operas that are cut) it would not be enough to over take the #1, La traviata, seen 4,190 times.
2. Die Zauberflote - 3310 (561)
8. Le nozze di Figaro - 2483 (545)
9. Don Giovanni - 2299 (473)
15. Cosi fan tutte - 1538 (364)
27. Die Entfuhrung aus dem Serail - 855 (163)
69. La clemenza di Tito - 302 (65)
74. Idomeneo re di Creta - 276 (62)
92. Die Zauberflote (reduction) - 223 (58)
108. La finta giardiniera - 194 (43)
164. Der Schauspieldirektor - 108 (34)
For comparison with Der Schauspieldirektor, other operas that saw 98 to 118 performances over this period include: Adriana Lecouvreur*, Medea*, Il viaggio a Reims, Orlando (Handel), Platee*, and Lulu. (Each opera marked with an asterisk was that composer's most performed work).