Something about a bass aria just makes me happy. It's a unique musical moment -- incredibly effective at moving the emotional burden of the opera along. The bass is almost always a "foil" -- either a villain or supporting role that is used to contrast the hero. Here are two of my favorites.
Most people have heard (or at least heard of) La Boheme. The bass role in La Boheme is Colline, the philosopher. The penultimate aria in the opera is sung by him, before the dramatic death of Mimi -- where he is preparing to sell his beloved overcoat (which is where he keeps his writing) to buy her medicine. In some ways, the aria is comical, and is supposed to lessen the emotional tension of the death scene to follow and of Rodolfo's anguish. However, it is itself a tremendous recognition of the transience of life -- and it's the bass which allows it to stay on that cutting edge of comedy and pathos. Listen for yourself:
The second is from Borodin's "Prince Igor." The bass here is the villain -- the Pecheneg Khan Konchak. He has taken Igor prisoner and now asks him why Igor is so sad in confinement even though he is treated so well. Konchak offers him any horse, any woman and even his sword -- of course showing off at the time. Konchak is the typical personification of 19th century "oriental" -- the seductive, cruel, swaggering easterner. Notice the way the bass allows the aria to shift from seduction to menace quickly. As he offers Igor his freedom in return for a promise to not stand in his way Konchak becomes a tempter -- reminiscent of another famous bass role, Méphistophélès in "Faustus." Igor's heroic baritone provides a powerful, simple contrast.
Of course, YouTube is not place to listen to opera. Listen to the whole thing sometime!