Gina, These are the operas I've seen:

La Traviata
La Boheme
Madame Butterfly
Cavelleria Rusticana/Pagliacci (always performed together, like the operatic equivalent of a double-header ;))
Rigoletto

I would have to put Madame Butterfly, La Traviata and Cavelleria Rusticana (which is the opera Anthony performs in GF3)with Pagliacci in my Top Three. Those are in no particular order, btw. Although Rigoletto has some wonderful arias, I always found it a bit depressing, and I always considered La Boheme sort of a ripoff of La Traviata. Here's something to keep in mind - the girl almost always dies!!