The Nintendo DS Lite (2006) is the final and newest revision of the original Nintendo DS family.
The DSi and DSi XL are generally classified as a separate platform generation (like Game Boy Color to Game Boy Advance), not mere revisions of the DS. They have exclusive software (DSiWare, DSi-enhanced carts) and lack GBA backward compatibility.
If you strictly mean "the last model branded simply 'Nintendo DS'", it's the DS Lite.