I have a few picks. 1st and my favourite is Shadow of the Colossus/Ico/The Last guardian by far. Even the original SotC on the Ps2 had an amazing artstyle and i still love it to this day.
I don't know about best art style since its very subjective, but Wind Waker is basically timeless because of its cell-shaded and minimalist aesthetic. So if its age or being able to stand the test of time is a factor in determining "best", then I'd choose Wind Waker. Easy.