Was more of a gradual thing, but it was probably a shogun or crocop... or wanderlei... or fedor fight. Those 2004 PRIDE fighters really caught my interest (probably because the mismatches and one sided beatings).
I know the first PPV I bought was Hughes Gracie (couldn't miss the mythical Royce vs. a new breed)
I think I'd go with these four
Fedor Nog, Shogun Arona, Crocop Wanderlei 2, Anderson Silva Rich Franklin.
I remember my dad ordering an old Matt Hughes PPV but I was "too young" to watch it. Those days it was more controversial I suppose.