Godzilla.
It is a little unfair though, as i have seen every Godzilla film and own many of them, where as i have only seen The original Ultraman, Toward the Future and Ultra Galaxy Legend, and only own one of the Grey market sets of Ultraman.
But i tend not to compare the two, thought they maybe in the same Genre, they appeal to different people. Plus it's a Film franchise vs. a mainly TV based franchise.
But no matter, i'd likely vote always for the big G anyway.