Who was the most Evil ?

It's difficult to measure evil. Perhaps one measure is to account for how many died because of them.

Hitler: All deaths of Europeans during World War II: ~45 million
(Doesn't count non-Europeans who were killed in Europe)

Stalin: Deaths by famine, gulag and other actions: ~25 million

Mao: Deaths by famine and other actions: ~30 million

Pol Pot: Everyone killed by the Khmer Rouge: ~3 million

Kim Jong Il: Difficult to determine. Perhaps ~3 million

Of course, just measuring the deaths, even if you have accurate numbers, doesn't take into account motivations, oppression and other factors that could be examined. The Nazi regime under Hitler was actively malicious, with an unknown number of deaths in the decade leading up to World War II. Take into consideration its policies, concentration camps and whatnot, and Hitler may have more points in the actively evil column.

The deaths attributable to the others were generally less motivated by maliciousness than they were by doggedly following ideological policies that were clearly failures. There was likely some degree of maliciousness in there, as well, but not to the extent of Hitler.

Also, this list is far from inclusive. It doesn't deal with the deaths in the Pacific theater during World War II and the period leading up to it; Japan was responsible for the deaths of ~20 million in China alone.

If we're limiting ourselves to just these five individuals, I'd have to vote for Hitler as the most evil.