If you’re making a list of the best 2d animated films of all time you’re thinking almost exclusively of Disney animation work (I know there’s a great deal of 2d animation that comes out of Japan but that’s going to be excluded from this conversation because not enough people see those movies). The Lion King probably belongs at the top of the list, at least based on audience and critical reception. It made a ton of money, it has enduring popularity, as evidenced by all the money it made upon being released again in 3D.
Beauty and the Beast was a 2d animated film so good it managed a nomination for best picture from the Academy. All the early Disney movies are considered classics and with good reason. They’re unbeatable (think Dumbo, Bambi, Snow White and the Seven Dwarves, etc) and they have stood the test of time to maintain a place in the hearts and minds of audiences over the years.