top 5 operas?

Tough to come up with a ranking because people's tastes are different.

To begin with start with Mozart's Don Giovanni, Magic Flute, and La Nozze di Figaro....

Eventually you should tackle Wagner's Ring and Tristan und Isolde, those are the top 5 for me, although tastes vary and you shouldn't quite start opera listening from Tristan, because most likely you won't enjoy it.

After reviewing the Levine and Boulez cycles on DVD I see that both of them have their strengths and weaknesses. One should start with Levine's but Boulez is better on the acting side I believe. It is just the sound and video quality that really kill Boulez edition.