The commonly accepted answer is that smoking is the worst, snorting the second worst and swallowing the "least bad" way of doing it. But honestly, I wouldn't touch that stuff with a ten foot pole. It fucks with people's minds. I do think that people who do ice and go absolutely positively batshit, are the people who were prone to mental illness before already. They just needed this extra push that sent them off the deep end.
Not all ice-heads are psychopaths, but chances are that, if you're already a bit off, that ice is going to make it a hundred times worse. That doesn't mean that it couldn't spoil a sane person though. If you consume ice and you do it regularly as a habit, it will screw with you.

One thing that I do find funny is that I've not yet met anybody who is addicted to ice. Doesn't matter if they do it every day and if you gave them five bucks when they're broke, they would rather buy drugs than food. They all say the same thing "No it's not a problem." "I don't do it so often." "Of course I'm not addicted." "I don't understand why people say it's addictive. I've done it for years and I could stop whenever I want." Those were all direct quotes. Sad delusional bastards.