Mine is Master Chief or any Sony owned character. Allow me to explain:
So far every third party fighter that has ever existed in Smash has had some sort of connection to Nintendo games or hardware, despite it sometimes seemingly only tangentially related. Sonic and Snake were the first two surprise additions to Smash, but both of those characters had games appear on Nintendo hardware prior to them being added to Brawl. Snake may be mostly famous for the Metal Gear Solid series but Metal Gear 1 and 2 were NES games and Sonic first came to Nintendo hardware on the Gamecube with Sonic Adventure 2 Battle. EDIT: And Sonic Advance on the GBA. Which apparently both came out on the same day in Japan?
And so it goes with every third party character since. Even if it doesn't seem that way they've all had some prior connection to the Nintendo brand. I would say the character that made a lot of people think "Wow, there are no rules" was Cloud, and technically this is an outlier seeing as (at the time at least) FF7 had never appeared on a Nintendo system. But even Cloud sort of tracks because if you take the Final Fantasy franchise as a whole obviously it has a very strong connection to Nintendo and Cloud himself was also appearing in spinoff games that were heading to Nintendo hardware so it's a vague connection, but it's still there.
Master Chief is something a bit different though. No officially released Halo games, there was a cancelled Halo DS game but I think it's fair to say that doesn't count. He's a third party character with no real ties to Nintendo hardware at all. As far as i'm aware the only thing you could grasp onto is that you can use Master Chief skins in Minecraft on a Nintendo system but that is some flimsy justification if ever there was one. So yeah, Master Chief would be my pick, or basically any Sony character for basically the same reason.
I don't think Master Chief will make it to Smash though. 3 Microsoft characters seems like overkill, he's a gun heavy FPS character which i don't think lends itself well to Smash and even if something like that would be viable Doom Slayer just makes so much more sense to fill that kind of slot.