I don't think it's fair to say that one pianist is the best in any musical sense. Certain pianists excel in certain styles of work. Certain approaches appeal to different listeners. It's all relative, except with regards to technique. Even there, what's more important? All the right notes, good phrasing, wide sonority, tone color, dynamic control, what? There is no definite yardstick.
Unless you can quantify artistry (which I have yet to see done) you really can't say any pianist is musically superior to another with absolute precision, except in the realm of pure technique (which is, debatably, the easiest part of pianism). Certainly, we can go on the basis of a concensus (i.e., most will agree that Argerich is better than the kid down the street), but even that will have it's flaws in terms of musicality, because it's so personal a subject.
When discussing the greats, saying that "Richter is better than Argerich" or "Horowitz is the greatest" or "Brendel is more intelligent than Schnabel" could be compared to saying "Chinese food is superior to Italian" or "The Beach is more beautiful than the mountains." These things are all great/good/beautiful to some. Thus, to insult someone's tastes in this regard, is akin to insulting that person's musical tastes-perhaps why pianists get so defensive about it. Accept that you have your favorites, and don't whine that "everybody's" favorite is Horowitz (even though everything I've seen indicates that such a statement is completely false).
Also, I wish to address this issue about criticising others' playing. Just because we may not be able to do what the greats could do, does not mean that they are above our criticism. It means that we should show respect to their hard work, dedication, and love for music. We are still free to criticise, as long as it's done in a thoughtful, constructive way that does not insult their personal musicality.
Anyway, sorry for the preachy tone.