Mine personally was the beating the elite 4 with a Rattata level 12 and a level 100 Magikarp , what was yours?

For me it was achieving a 50win steak at the Battle Frontier with my Gardevoir back on Emerald version. Now even a Shedinja could stop her.

Beating Black Version for the first time. It and White had a very compelling story and really made me feel something for completing it.