I just wonder how in such a subjective field the verdict on a concrete top ten can be so unanimous among critics.
The obvious answer is that Shakespeare, Dante, Homer, Tolstoy, Milton, etc... simply are just that good.
I have to question how nearly everyone comes to the same conclusion.
Is it really that difficult to fathom that Shakespeare, Dante, Homer, etc... Or in music: Bach, Mozart, Beethoven... Or in the Visual arts: Michelangelo, Rembrandt, Rubens, etc... are acknowledged as among the greatest within their respective fields by such a number of individuals because they are? Your struggle seems to be: "Shakespeare, Dante, Homer, etc... don't seem to me to be the greatest authors I have read... and since it cannot be possible that what I like and what is "great" are no one and the same... everyone else must be wrong... or must be blinded by the opinions of others."
Lets suppose we exist in a world where nobody has access to the opinions of others, say everyone lives in a small room with every work currently considered seminal in the western canon and as many years or lifetimes as needed to finish them all. They enter the room with their current memories of experiences, upbringing etc, minus anything they have ever read and minus what they know about Western literature. After concluding their lifes work how many of them would come out and say "well my absolute favourites were William Shakespeare, Dante, John Milton, Homer and Chaucer? I suspect very very few and that the actual results would be exceedingly sporadic, with authors many of us won't have read or heard of appearing everywhere in peoples top 100 or what have you.
The problem with your scenario is that we don't live in a void. We live in a world in which Shakespeare and Beethoven and Michelangelo are part of our collective cultural memories. We cannot read Milton, Goethe, Proust, Joyce, and many, many others without sensing the impact of Shakespeare. We cannot look at Caravaggio, Rubens, Beckmann... or even Lucian Freud without recognizing elements of Michelangelo. Art involves a dialog with art... as well as with life.
Now I know it's often a partly historical context which places these particular writers at the top of the heap, and that's fair, but it's definitely true that certain crtics like Bloom will state that Chaucer's writing specifcally, as distinct from his influence, is second only to Shakespeare's and that Miltons writing is third, etc. I've read Chaucer and I don't pretend to be an expert but that just seems untrue to me and I question how many people (in universities especially) are expert enough to actually make a critical judgement on Chaucer's writing - especially given that it's middle english. But you'll get Bloom going on about the strength of the writing and evocativeness and all that and it just feels like wind to me...
You admit that you are not an "expert" but then you turn about and question the opinions of those who are "experts" as being nothing but wind. Certainly no one would suggest that critics are always right... and certainly some critics are better than others... but the reality is that the "expert's" opinion is based upon a great deal of experience. What do you find failing in Chaucer? The author brings the concept of a unique voice as mirrored in the language and style of each character. This results in an invention of character that surpasses a great many writers. His language, itself, certainly strikes me as quite evocative and marvelous. Quite honestly, his "Middle English" is not all that difficult to grasp... certainly much less so than Langland's Piers Plowman. But lets be honest here... do you imagine that Faulkner, James Joyce, T.S. Eliot, Proust, the epic poems of William Blake... or many other works of more recent literature are far easier to grasp?