I visited the Holocaust museum in Washington D.C.
It remains one of the most moving/terrible things I ever encountered.
I still have it burned into my little brain (I was horrified) where I saw a massive pile of shoes taken from people.
It's difficult to say whether Hitler or Stalin was worse (Stalin wins on body count, UNLESS you include WW2's casualties) but the sheer mechanized destruction of life for something so pointless is justifiably one of the more insane things we've had in the entirety of human history.
I recall as a child, vividly also learning that while Jews were the most famous victims they were far from the only too.
Gypsies, dissidents, homosexuals, and Slavs - the sheer breadth of Hitler's victims amazes me.
"Never again." should be not only the mantra of Jews but everyone. We needto look at these horrors and have them burned in our psyche so we never ever let the kind of hate which motivated them take place again.
(To this day I don't know if we'll ever know who knew what in terms of who was involved in the Holocaust)
EDIT:
In a black bit of humor, do not type "eight" with a ) when you're describing the age you visited the holocaust museum or it'll come up as a

. That was damn creepy.