I have to agree with Hendrixcat, Sneider Weisse followed by Paulaner for me. I like most other Weissbiers too, though I think Erdinger is a bit bland, so I'll only drink it when I'm stuck.
I have heard good things about Ayinger, but have yet to get my hands on a bottle.
I have yet to find a decent Weissbier brewed by an Irish company. The Franciscan Well do a nice wheat beer, but I don't think it tastes like a Weiss.