My opinion has changed. Considering the shiny Yveltal I got from Wonder Trade was an obvious hack, it's far less than the best I've gotten from Wonder Trade. The best is probably either the shiny Forretress I got on the same day, the shiny Spiritomb with great nature and moves, or the Bagon that was one digit short of winning the in-game lottery and got me a Rare Candy like two days ago.
A shiny Skarmory, which was on Lv. 100. I was just confused when I got it. Also I'm not sure if it's a cheat or not. I just put it in the box to my other shinys.
Shiny Petilil was definitely the best, but I got a shiny Regigigas (obviously non-legit tho, it's in a Love ball), shiny Sylveon (but I already have one lol), and a Hoopa the same day. It was a wild day for wondertrading, before that I never really got anything good. Hoopa is the second best thing cuz I didn't have one.
I'm not sure which one is "best", so I'll say my favorite. Shiny Jirachi. It obviously was hacked because it's a Japanese event with the ribbon removed. Plus it has the English name instead of Japanese. But I don't care because it's my favorite Pokemon.
Edit: I just received a Shiny Arceus, but I traded it back because it had no name. I had a bad experience in Platinum with a blank OT Shaymin crashing the journal that was supposed to say I got it off the GTS. You know that journal shows up when the game is started? Yep, I can't start my Platinum game. I don't want anymore blank names causing freezes.