Holocaust deniers are an interesting sort, if by "interesting" you mean "have their heads in their asses". The persistent theory is that the six million Jews (and about six million non-Jews, including Gypsies, homosexuals, handicapped, and political dissidents) that disappeared during the Second World War were actually just part of the masses of displaced persons that were lost in the postwar shuffle, many perhaps trapped behind the Iron Curtain. To believe this you also have to believe that the photographic evidence was all forged, that the confessions of camp guards and testimony of survivors were all part of an elaborate hoax, and that the Soviets who couldn't make a tractor work properly were capable of keeping a secret as big as several million refugees hidden under assumed names. Plus, you'd have to believe that the gas chambers found after the war were actually used to disinfect lice-ridden clothing and the crematoriums were used to burn this clothing (not sure why you'd first disinfect and then burn). Yet another theory argues that sure, a lot of Jews were killed, but not a full six million--maybe ten percent of that.
However, I don't really see the point behind the denials. Even if you were to agree with the idea that far fewer Jews were killed (or that they were merely forced into camps during the war, then displaced as refugees after the war), it still counts as an atrocity suffered by them. Would things be okay if it turned out that the numbers were off? If only 600,000 were killed, would we be able to write off that chapter of history as no big deal?
The other thing is, why pick on the Jews of all people? Last I checked, they don't riot, rarely commit violent crimes (Meyer Lansky aside), and for most of history had no country of their own so hadn't invaded or enslaved anyone (okay, I'll accept the Canaanites and Edomites hating them, but that's it). There are many other groups of people that the hatemongers could pick on with more plausibility:
1) The Italians. Christians can blame Italians for killing Christ. Jews can blame Italians for tricking the world into thinking it was the Jews who did it.
2) Russians. They embraced Communism, created really bland food and are emotionally distant. Plus, they invented Polack jokes which should piss off natives of Pittsburgh and Chicago.
3) The French. No need to explain further, but they are responsible for everything awful in history. WWI--they pick a fight with Germany and then try to drag everyone else into it, then after that they act like dicks and make the Germans go Nazi and kick their asses again, then we have to bail them out. They brought us Napoleon (though the Italians can be blamed for his heritage), who invaded and attacked everything, they messed up every colony they had and created dysfunction throughout the Third World, and they handed us Vietnam. Plus, they're the reason the whole world isn't driving on the same side of the road.
4) The Germans. They go and ruin a cool thing like the Roman Empire, they invent Protestantism which caused all that religious strife for centuries, they (Marx and Engels, two GERMANS) invent communism, and when that turned out not to be obnoxious enough they invent Nazism, and they bring us really lame music (99 Luftballoons? Ach, der crappen!).
Not that I'm advocating hatred of any groups, but with rich targets like these, why go through the mental contortions of picking on the Jews? All you can really blame them for is musical theater.