The standard seems to be Illustrator but I really love using Flash to draw all of my vector stuff. It feels more natural for me.
Usually I use Illustrator, but sometimes I jump in Flash and mix it up. Funny thing is I learned Illustrator back in the early 90's and didn't do much with it. Then in the late 90's I taught myself Flash and that sort of got me back into Illustrator over the last decade.