I’ve been asked that question a zillion times over the years and it always comes down to two factors, both of them personal: 1. Do you like the sound the piano makes (different brands have different sounds, obviously), and 2. Do you like the touch – how it feels to your fingers as you play.
That being said, I would also advise anyone to stay away from the off brands, of which there are many. My personal preferences are Steinway, Bosendorfer, Kawaii, and Yamaha, although there are certainly other good brands — but those are my personal favorites.
(We’re talking about acoustic pianos here — not digital pianos — that is a different subject we will take up another time.)