I'm an amateur pianist and have lots of experience in pianos, I know and have worked with some of Germany's finest klaviermeisterbau.
In general Steinways are a safe bet but I would never go so far as saying they are the unrivaled best. They are simply consistant. Some friends of mine recently invited some proffesional pianists to play and compare a Steinway D and and a Bosendorfer Imperial grand. Without exception they and the audience agreed that the bosendorfer sounded the best, being mellower and sustaining the tone the longest. However the pianists found the Steinway much easier to play. Personally I agree, as bosendorfers feel as though they have very litte felt unter the front of the keys, and therefore feel hard.
What I have to point out is that if you want a TOP quality piano at a tenth of the price, you cannot go wrong with buying an old piano, provided that it is in suitable condition, and having it rebuilt. This is especially the case with old German brands such as IBach, Bechstein, Roenisch and Feurich to name a few. Unfortunately these piano manufactirers have all lost their individual sounds now as it's less risky to sound like a steinway as the public believe his to be the correct steinway sound. Luckily it is possible to buy top quality non-doped (old fashioned) hammers from Abel. The important thing is be prepared to spend some money and only hire very experienced restorers as only they can voice pianos properly. This is the process also known as intonation, involving needling the hammers so they they sound mellow at pp and bright at ff. Sadly in my experience most english technicians thing this is black magic. It isn't, it's just very hard to train yourself, and makes all the difference between an OK piano and magnificent piano.
Importantly, a fully restored piano will sound better than a new factory finished piano. This is because factories have to limit the time they spend on each instrument and explains why even steinway send their technicians to the concert halls to bring them above factory standard.
I'm currently having a Feurich-Welte restored for my personal use. The piano, a grand, cost 5000 euros and the restoration ~8000euros. This is an exceptional instrument as it contains a Welte pneumatic reproduction action. This was invented in 1904 and I have rolls of Debussy, Ravel, Scriabin etc playing their own music. Unlike a player piano this plays the notes at exactly the right time WITH dynamics. Sadly most recordings of these instruments do not do the system any justice as it can only be as good as the pianos it plays and has to be perfectly regulated. I've heard many of them but only one perfect one and it was astounding. I've spent the whole summer rebuilding the action near Bonn with some experts, re-covering bellows and making new valve pouches, and hope to have the system completed, installed and regulated within the next year. If anyone is interested in knowing more about the Welte system please ask!
Best wishes to all ,
ps: the best piano I have ever played is a Grotrian Steinweg concert grand, in terms of touch and sound.