which is best to get? thanks

Each of his albums are more or less different from each other, and many fans have their favorites, so it's hard to suggest to you which one's his best, although he did peak (as far as the mainstream is concerned) with "Antichrist Superstar".

But I would just pick up his "Lest We Forget" best Of... CD and whatever songs you like on that, find out which album they are from and pick that up.
Or just start at the beginning with "Portrait Of An American Family".

thanks