No doubt, one of the best painters of the 19th century, if not of all times.

Nevertheless there is one observation that really strike me while visiting an exhibition with 150 of Turner's works here in Germany (City of Essen, Folkwang museum): I noticed that most of his paintings are really "tourist-like paintings", i.e. when showing a landscape, a city or even the burning of the House of Lords he has always chosen the angle for his paintings, today's tourists would choose themselves for their photographies. Every time I see an english landscape painted by Turner I imagine a bus with 50 tourists, stopping exactly where Turner stood, taking out their cameras, crying "ahhhh" and "ooooh" and (if they are americans :) "grrrrrrrrreat", while taking dozens of pictures.
Once again, don't get me wrong: I don't say that he wasn't a good painter, I just say that he's the same for painting what Goethe is for literature: a perfect and intelligent tourist.