I don't think holocaust denial should be a crime. Yes it's despicable, but I'd rather put it in the same category and hollow earthers and UFO spotters. We can't legislate people out of their own stupidity. It's not going to stop people holding such views or circulating them among themselves anyway, so it's utterly pointless.
For me Porn is easy too. If it only involves consenting adults, it's fine. If it doesn't then other crimes have been committed and distributing it is de facto complicity by association in those crimes.
That only leaves slander and libel. These are civil, not criminal matters and should be prosecuted as private matters based on the harm done to the individuals so slandered or libeled
I'd add incitement to commit crimes as an issue. But that falls under complicity or conspiracy to commit a crime. If the speech in question doesn't cross that line, then it should be free.
Anything left?