Your first pokemon event.

Think mine was mew for 3rd gen.

Can't remember exactly as it has been awhile.
The Manaphy event from way back in 2007. I think that was the only way to legitimately get Manaphy in any way, afaik.
Hm...oh geez that was a while ago...

I think it was Darkrai over Wi-Fi. My first "Go to this store and download the Pokemon" was the Arceus event. My little cousin got it with me as well....and proceeded to own the Elite Four with it. I told him that was really cheap.
You can't use ANY event Pokemon at the Battle Subway anyway. Likewise, certain PWT tourneys outlaw the usage of event Pokemon as well.
I never went to a Pokémon Event, because:
- Pokémon Day: Way to far away.
- Pokémon TCG: Not my interest.
- Pokémon VGC: Not in my country.