In answering the question "What are the top 5 opera arias?", we're faced with an immediate problem: how to define "top". Simply taking it to mean "best" makes things harder, as defining the best arias will be purely subjective. A more realistic approach would be the democratic one: the top 5 preferred by the opera-listening public as a whole, or the top 5 preferred by singers. And perhaps the best way of assessing these is to look at which arias have been the most recorded. To do this I've gone to arkivmusic.com, a mail-order source of classical recordings that lists how many recordings are available of any work of music.
The results (as of June 20, 2007, at least) reveal, not surprisingly, that the top 5 arias are all from Italian operas. Here they are:
#1. "E lucevan le stelle" from Tosca (Puccini). In prison awaiting death, Cavaradossi (tenor) recalls a night with Tosca.
#2. "Che gelida manina" from La Boheme (Puccini). This is sung by Rodolfo (tenor) on his first meeting with Mimi. He tells her of his life - he's a poet. By the end of the aria he is deeply in love.
#3. "Nessun dorma" from Turandot (Puccini). Calaf (tenor) is keeping his name secret, and he has agreed to be killed if the princess Turandot learns it before morning. Although all of Peking is trying to find out his name, Calaf is supremely confident.
#4. "Un bel di vedremo" from Madama Butterfly (Puccini). Butterfly (soprano) has been waiting three years for her American husband Pinkerton to return, but she hasn't given up hope. She dreams of him arriving on his ship and climbing the hill to her.
#5 "La donna e mobile" from Rigoletto (Verdi). The Duke of Mantua (tenor) cheerfully warns of the dangers of fickle women.
You'll notice that these are all from Italian operas, that 4 were written by Puccini, and that 4 are for tenor. So this list doesn't tell the whole story, not by a long way. But I think it does represent what most of the public thinks of when they think of opera.