Latios.
My reasoning? I've never caught him :P
But wait, before you slag me off. Here's my story:
On Emerald, I chose "blue pokemon" on the t.v. thinking it would unlock Kyogre :P
Went to Cave of Origin....nothing. Then I went to the Weather Institute. I got told it was sunny somewhere, which I figured meant Groudon...I was confused. I then went outside, and wandered into some grass accidentally. I met Latios. I selected an attack, it avoided Earthquake with Levitate, then fled. I looked on the map, started chasing. I eventually even got a pokemon to use Mean Look + Baton Pass (Smeargle). I had 50+ encounters with it, but could never catch it. I'd not used my Master Ball, I just thought that looked ugly.
So, when I downloaded my Enigma Crystal, I actually just...didn't want to battle it in Pewter City, so turned around and went the other way. It's still sitting there now.....I'll reset and get another Enigma Crystal before the event ends, though.
Yeah, so I'm like Eusine, but with a Latios, and not as...well...weird.