To set the context of this question:
A target which meets the criteria to be listed in the answer would be
- An entire species, or
- The entire population of an organisation/state/civilisation/similar entity which occupies territory spanning across at least one planet.
The genocidal campaign need not necessarily be considered successful in the Empire's books, though it would be appreciated if such cases are highlighted (since failures by the Empire of such magnitude would probably be rare and notable).
The timeline to be considered should begin from the Declaration of the New Order, when the Galactic Empire officially came into being, up to the official end of the Galactic Civil War when the Empire/Imperial Remnant signed the peace treaty with the New Republic in either canonicity.
We know Alderaan was destroyed (though at least in Legends there were a meagre few survivors, mainly those who happened to be off-world). The Geonosians were also wiped out in canon and I believe in Legends too. Besides these two, who else are there?
I would like to know mainly about those in Legends, although if I missed any out from Disney canon please feel free to mention them.