As with any 'greatest' lists there is no one correct answer. If you put your question in a search engine you will find many different lists, with people listing their own personal favourites as well as more serious attempts to look at each musician's legacy and importance.
As you can see from looking at these lists - this one is quite typical - you can't get five names that everyone agrees on, but you can find certain names that keep coming up. Pianists that are very highly rated by the public include Herbie Hancock, McCoy Tyner, Art Tatum, Keith Jarret and Thelonius Monk.
You can find a more detailed and in-depth discussion of this topic here - the author gives the top place to Art Tatum (with the more famous Fats Waller in 7th) but talks about all 10 of the greatest jazz pianists (in his view) in a lot of detail.