Who is history's most influential vegan/vegetarian?

I'd say Leonardo da Vinci. Or Albert Einstein.

Hitler, I have to comment since that old myth popped up here, was not a vegetarian. That was propaganda, but the truth is that quail and ham were some of his favorite foods. He ate an almost vegetarian diet for brief periods due to stomach problems.

Influential in what context? If you mean "influential" that they encouraged people to adopt a vegetarian diet, it would be Gautama Buddha.

Gandhi was more influential as a peaceful political revolutionary than as a radical vegetarian. Most people would refer to his peaceful protests as his greatest achievement rather than his diet.

Thomas Edison

Leonardo Da Vinci

The Beatles

Aristotle

Mark Twain

Ghandi was a racist and a womanizer. Hitler wasn't vegetarian. Pam Anderson is nothing special unless you lust after large breasts and sexist campaigns for welfarist organizations.

Probably the most influential vegans would be the various cells and individuals involved with the ALF because they actively go out and liberate animals and put their lives on the line to do so.

Ellen Degeneres

Ghandhi

There were many. But I'd choose Pythagoras.

Gandhi or perhaps Pythagoras

Gandhi hands down.

Ghandi ate meat until he died. I can't think of one mostly because most haven't reached the age of 50.