Very tough choice. “Che gelida manina” would be one. “O mio babbino caro” is another. “Dove sono.” “Quando men vo.” “Di quella pira.” “Di Provenza il mar, il suol.” I can’t do it, can’t pick one. (And how could I forget “Velia”?)
Actually most of my favorites are not arias but duets (“Au fond du temple saint,” “Ai nostri monti,” the Flower Duet from Lakme), quartets and sextets (e.g., from Rigoletto and Lucia), ensembles (“Miserere” from _Il Trovatore), and big choruses (Hebrew Slaves from Nabucco).