I've looked back at most of this thread and I have a couple of points on my own personal viewpoint:
1. The Holocaust (and the murder of so many others, the slavs, gypsies, homosexuals, the mentally ill, etc) is undisputable. There is a whole host of evidence to support it as pointed out by other posters.
2. Trying to compare other genocidal acts is pointless, they are all barbaric whatever the reasoning behind them.About the only way that you could say that one was 'worse' than another would be to base it on the percentage of the 'target population' was murdered. Saying that, IMO, the Nazi slaughter was 'worse' because of the industrialisation involved, the numbers of people actively involved in carrying out the slaughter and the relative calmness of the process. The Darfur and Rawandan genocides are far more passionate in the killing (a poor choice of word and probably difficult for modern westerners to understand properly).
3. I feel that the reason why denial is so bad is that it may lead to the view that such horrors may be acceptable again. Our society may seem to be terribly civilised and strong, yet it is in reality very weak. Anything in the long chains that supports our society could fail and plunge us into chaos. Such chaos would provide a ripe adience for those who might advocate such measures. After all, this was the situation that led to the rise of the Nazi party in the first place.
4. A lot has been missed on the thread about the activities of Stalin, he presided over a state of terror every bit as bad as the Nazis and just as industrialised. It was just further afield than the Nazi camps. He was responsible for the slaughter of 25% of the Ukrainian population, killed more than 2 million muslims in the souther territories of the USSR, had over 5 million people incarcerated in the gulags.
5. China is a very odd place in this respect. The government is very vociferous in wanting Japan to face up to the crimes committed against China and has owned up to purges under Mao's rule. Yet they are very coy about their continuing actions in Tibet and in smothering dissention in the country.