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.

I can't make a decision on 5 operas, so I have picked my 4 favorite composers of operas and their works that I enjoy most:
1. Puccini: Tosca; La Boheme
2. Verdi: La Traviata; Rigoletto; Aida
3. Mozart: Don Giovanni; The Marriage of Figaro
4. Wagner: Gotterdammerung (or the entire Ring Cycle if you consider it as a single work); Tristan und Isolde

I'll gear mine for the novice. You'll enjoy these more:
1) Orff - Carmina Burina - untouchable
2) Carmen - Puccini - many selections you know
3) Gianni Schacchi - Puccini - hard to find, but worth it
4) & 5) Sdcampbell's Wagner and Verdi selections