As the others have said already, it depends what you are after.
If you are looking for a large specimen floating plant for an open topped aquarium/indoor pond, then the Water Hyacinth, Eichhornia crassipes, is a very impressive plant. Various other and smaller floating plants are available, ordinary Hornwort, Ceratophyllum demersum, does very well as a floating plant, Crystalwort, Riccia fluitans, is very nice.
For plants growing form the bottom but producing floating leaves/flowers, the many Nymphaea species are superb tropical water lillies small enough to be grown in a large tank.
More info!