Perhaps the greatest Mozart aria is Soave sia il vento, from Così fan tutte. That much I can say. Actually the only one the that I can think of that rivals it is Dove Sono I Bei Momenti" from Figaro.
In terms of opera though, Puccini is just as good if not better than Mozart ("E lucevan le stelle" from Tosca is quite possibly my favorite aria, although I can't say this for sure). The two hold dear places in my heart. I am also fond of Fidelio, having listened to it several times over the last month... it may not be as great as some other operas, but there is something sublime about the way Beethoven handles music in general which just clicks with me.
Unlike J.L.G., my first experience with Opera was Otello by Verdi, which I did not like and which put a soar taste in my mouth towards the genre for ages. It wasn't until I discovered Turandot (Puccini) that I came to appreciate Opera (and write one), therefore I may have to place this opera or La Boheme at the top of my list.
Ah! It hurt to write that, but there it is.
Ranking music is always a pain!!