Joffrey Baratheon.
Or as I like to call him, Prince Douchewhore of Doucheington, Lord of the Cocksucker Realms and Defender of the Asshole Empire.
It is, quite literally, impossible to have this discussion if we're even going to pretend someone wins out over Joffrey. This thread should be "AFTER Joffrey Baratheon, who is the second biggest douche?"
Sauron and Sauruman at least had PLANS, they had some goal besides being simply evil, something to aspire to and attain. Cirsei too, and ditto for most "evil dictator" types. Even Tygra, and by the way I agree his "tragic past" doesn't excuse him being a completely worthless human being, but even TYGRA is not just outright, no bullshit, no reason or rationality EVIL.
Joffrey has no goals, no tragic backstory, no redeeming quality whatsoever. Except maybe his magnificent head of hair...but ALL the Lannisters have that, so fuck him! Just to give you an idea, Jame Lannister, his father, is an incestuous asshole who murdered a teenager and tried to kill and eventually crippled a child and he's FAR LESS of an asshole than Joffrey Baratheon.
Fuck.
That.
Kid.