I turn on to natural. The silicone only induces a kind of scientific detachment and esthetic appreciation.
And since eyes/face are usually my first landing-spot, that plays a part. I don't care that someone has a cawubunga bodaciousness to her if her above-the-neck looks don't please me first.
So my list for big-hootered ladies would run (in no particular order):
Nadine Jansen
Ewa Sonnet
Yulia Nova
Ines Cudna
Sharday
Anna Ohura
My non-natural list (wherein the boobjob doesn't just hit me as artificial) are the Tera Patrick, Veronika Zemanova and Veronica Black (she has other names, but that's the one I remember - nice ice-blue eyes).
Some truly spectacular natural boobs of recent times are Princessa at MCNudes and Merilyn (but she has to learn to stop that thing she does with her mouth - kinda goldfishy, if you know what I mean).
My 2 cents