According to ‘172 of the world’s leading opera singers’ polled by BBC Music magazine, the all time greats are:
1 Marriage of Figaro
2 La Boheme
5 Peter Grimes.
Well, the sample is rather small and anyway, professional singers stand too close to the flame to judge the heat.
A list of greats in which Verdi and Wagner figure first at 9 and 10 cannot be representative.