First row: Sardār Kamāl Khān, Aristotle "Aris" Kristatos, Sir Hugo "Hugger" Drax (Senior-Lieutenant Hugo Graf von der Drache), Auric Goldfinger, No. 5 (Colonel Tov Kronsteen), No. 2 (Emilio Largo), No. 1 (Ernst Stavro Blofeld), Dr. Julius No, No. 3 (Colonel Rosa Klebb, KGB), Donald "Red" Grant, Mr. Big (Dr. Buonapart Ignace Gallia Kananga), Francisco "Pistols" Scaramanga, Carl Stromberg
Second row: Lieutenant-General Orlov, CA, Gobinda, "The Chief" Brad Whitaker, Oddjob, Franz Sánchez, Charles Wint, Irma Bunt, Albert Kidd, Maximilian Zorin, "Tee Hee" Johnson, Baron Samedi, Nick Nack, Jaws (Zbigniew Krycsiwiki)
Third row: Elliot Carver, Renard the Fox (Victor Lavrentievič Zokas), Elektra King, Lieutenant-General Georgi Koskov, KGB, Milton "Milt" Krest, Dominic Greene, Adolph Gettler, Mr. White, Le Chiffre, May Day, Major-General Aleksei Anatolovič Gogol, KGB, The Whisper, Zao, Sir Gustav Graves (Colonel Moon Tan-sun, KPA), Ksenia Sergeevna Onatopp, Janus (Major Alexander "Alec" Trevelyan, RM, Ret.)
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Necros? The guy threw exploding milk bottles and strangled people with headphones playing 80s music to their death rattle! He single-handedly took on an entire safehouse of British agents and probably inspired a bit of 'Hitman' while doing it. How's that not memorable enough?
SPECTRE was quite an organization, although not of these characters were part of it. My favorites were Red Grant, Emilio Largo, Xenia Onatopp, Fiona Volpe, Helga Brandt, Ourumov, Ernst, Stavro Blofeld, Dr. Julius No, and Dr. Kaufman.
Kingwilfre is right, Ouromov shoulda been there. And all versions of Blofeld. And as a request, I'd like to see an honorable mention: Jonathan Hunter.