Namibian genocide against the Heroro, Namaqua and San peoples
Holocaust against European Jews
Porajmos against Romani (gypsies)
mass killings, deportations, child abductions against (non-Jewish) Poles constituted genocide too
Czechs were treated less harshly but also faced brutal reprisals, however likely only the child abductions that happened there too (afaik) constituted genocide
mass killings of Soviet POWs probably wasn't genocide in the legal sense, but there were definitely genocidal plans against the peoples of the Soviet Union