Of course the mass-murder of Jews (and Eastern Europeans in general) in whatever form is well-substantiated, what most revisionists are interested in is the details. The number of dead for example fluctuate with a few million depending on which historian you ask, also among Israeli historians, and if there was a grand scheme of extermination or it just happened for whatever reason (hunger, disease, slave labour, random killings - which are all undisputed), and if so when this was planned and by whom and how it was carried out.
There's been a ton of post-war fabrications (like soap and lampshades made of dead Jews and numerous document forgeries), unreliable eye witnesses, squeamishness, industry and interest group bias and lack of clear-cut Final Solution documentation to muddle the picture, but for anyone interested I can recommend the court transcripts from the Irving vs Lipstadt trial, after Lipstadt had called David Irving a holocaust denier in a book and he consequently made a libel case against her and Penguin publishing (putting the burden of evidence on her), and destroyed his own career in the process. Here the prosecutor and defense goes through much of the available evidence, and usual points of contention. If I remember correctly, and it's been many years since I've read up on this topic, one of the better sources for numbers were German population census before and after the war. The sheer volume of evidence hinting at orders for the Final Solution (on the background of all the other evidence from the war years), if only implicitly, are also enough for most serious historians to consider it a historical fact, within the usual parameters of historical uncertainty.
If anyone find the transcripts a bit dry I know there's journalists who've described the trial in some detail.
http://www.hdot.org/en/trial/transcripts/
