At least make a special focus on girls who actually are pettanko (as in they make a huge deal out of being flat) if you're going to ask for that, rather than girls who simply are flat.

1) Lina Inverse (Slayers) - Envious of most (if not all) of the other female cast members, delivering violent retribution for those who mention it. The first pettanko I ever knew, before I even knew what that word meant. Supported by a certain cast member who makes a particular effort to fire her up about it.

2) Mahoro (Mahoromatic) - An android who actually attempts to requisition bigger breasts or somehow make them grow. Her obsession is but a small but important facet to the cornerstone of her character ("Ecchi na no wa ikenai to omoimasu").

3) Mayumi Thyme (Shuffle) - The pettanko that made me actually learn the term. Unlike so many others who find it to be a sore spot, Mayumi takes great pride her in flat chest (while still mentioning it quite often), rationalizing that it puts her in a niche that will be sought by certain men. This makes her a rare and special breed of pettanko.

Runner up - The amusingly named Nymph (Sora no Otoshimono)