Possibly the biggest, baddest, meanest Marvel Universe villain and even the Mighty Avengers and Adam Warlock acknowledge him as the most formidable threat to the universe. He has enough power to swat aside the strongest heroes (Drax was created to combat him) and has proven to be very hard to terminate. To make things worst for humanity, Thanos is also incredibly intelligent and powerful.
Thanos does not want to rule or take over the universe. He wants to be its superior entity and does everything for his love of Death. He murdered half the universe to pay tribute to his lover (Death), killed Adam Warlock, murdered an entire planet to acquire the Designate (process of elimination), made a device to extinguish the sun, murdered billions of lifeforms and committed genocide to acquire the infinity gems. He only loses when he subconsciously wishes to lose.
However, there were times that he sided with heroes to save the universe or to protect his beloved death. The term "Madman" is not enough to describe Thanos of Titan. Easily worst than DC's Darkseid...and may be the ultimate insane stalker for the cause of love.
As a villain, he is easily one of the most evil..but I like him since his villainy provide excellent stories.