Why, why, why this insatiable need to categorize, make lists, prioritize????? I just don't understand what good it does. Someone, PLEASE, tell me!
It's just human nature. People like to organize their thoughts, especially in such a vast universe as opera's. Lists of masterpieces can actually provide guidance to people who are beginners, so that they can get exposed to the most significant works - of course, then, they'll make up their own minds; like some, dislike others. For example, I personally can't understand what people see in I Lombardi; I don't like Madama Butterfly very much, and while I love the music in Parsifal, I can't stand its libretto. All three are in most lists of so-called "top 100" but not for me (there are many highly regarded operas that don't do much for me, and more obscure operas that do - I just picked these three as examples). So yes, lists are only a starting point, but they can be useful starting points.
I think that even when we don't formally write down a list, we do keep a sort of "internal, virtual list" of our favorites.
When I hear or see a new opera (for me), I usually kind of attribute a personal score to it... A+... B-... C...
I know it's silly, we're not in grade school any longer and there are so many intervening factors that may make me like or dislike a given performance...
But I still do it, because it provides me a sort of record of what is worth going back to, and what is discardable and I shouldn't waste any further time with it (which only defines my own taste - someone else may love the ones I consider to be Cs and hate the ones I score as As, and there is no intrinsic judgment of value; I just rank what works for me and what doesn't).
Sure, then later I may actually completely change my mind and knock down in my preference something I had liked, or rediscover something I hadn't... but at the very least, I'll have some fun looking back at how I reacted to a specific opera the first time I was exposed to it.
Case in point: Rossini's Armida. I was completely blown away. I loved it! Ranked it A++ in my mind. Then later, I thought... meh... it's good but not *that* good.
The opposite happened with Lakme. I first thought, meh, C+ at best, there are some interesting arias but all very loosely connected with weak orchestration; a collection of good arias doesn't a good opera make, since it's not supposed to be a concert, after all. Later a friend asked me - how many numbers have you actually liked in Lakme? I looked it up in the track listings and said - this one, and this one, and this one, etc - and ended up with a large number of them (I don't remember all the details now). Then my friend said - OK, so, in this opera of a relatively short lenght, you liked this many pieces, and you still couldn't get yourself to like the opera??? Then I thought, oops, OK, maybe higher than C+... and with some revisiting I ended up liking Lakme, although I still see problems with it and is not one of my favorites.