all your questions are completely subjective. It would be pointless to give you my opinion because it may differ so completely with yours. For example, I think Maria Callas was an amazing actress, but I deplore her vocal technique. ( Duck, the tomatoes are coming in my direction)
Certainly, there are many operas that are on an unwritten Top 10 List, as they are played and replayed over the years.
La Traviata,, La Boheme, Marriage of Figaro, Magic Flute, Don giovanni, Hansel and Gretel, Flying Dutchman, Tales of Hoffmann, Carmen, and Lucia di Lamermoor come to mind, but there are hundreds more. The only way to decide on your "greatest" is to start listening, and keep on listening.
Same goes for voices. There are people who adore coloratura sopranos, and people who only like tenors. How to find the right mix? Keep on listening! What constitutes a good singer, in your mind? Is it purity of tone, clarity of articulation, those amazing high/low notes? Definitely subjective. One of my favorites has a voice that is robust, sturdy, will peel the wallpaper off the back wall of the theater, and is ugly. (It's as if Charles Bronson's face were translated into music) What he did with it was truly god-like, but that's only my opinion, see?
There's so much music out there! Go wild!
You could spend the rest of your life trying to answer your own questions, and enjoying yourself thoroughly
Best wishes