Personally, I say Splatoon. Why? Well first lets question Nintendo's sudden involvement in APEX. Nintendo(at least Sakurai) have historically been against, or at best, neutral towards competitive play(barring the Nintendo World Championships, and the weak ass Smash4 tourny they hosted). Now, all of the sudden, Nintendo is sponsoring APEX and becoming more involved in the competitive side of their games. Why the change? Well, I remember a while back ago Nintendo stating that they are seeking to recapture the hardcore game audience. Smash 4 is turning out to be quite competitive and even starting to bring over some of the melee players/audience. So this brings me back to Splatoon and what I personally think Nintendo intends for that IP. A competitive shooter. Yes its colorful, yes its a Nintendo game, yes its cartoony, but so is Smash and look at the competitive community that popped up around that game. Nintendo has announced that there will be a 3v3 online mode for Splatoon, but why emphasize team play in a shooter? Well its simple, Splatoon's objective isnt mass kills, the objective is map coverage. This style of gameplay naturally lends its self well to team play, where organization and strategy trump twitch reflexes and high kill streaks every time. I personally expect us to see more competitive involvement with Nintendo in the future. Nintendo Direct's have done great things for the company, so hosting an event like this or even hosting it on twitch is basically just mass market free advertising to the 'hardcore' market.
“What I say is, a town isn't a town without a bookstore. It may call itself a town, but unless it's got a bookstore it knows it's not fooling a soul.” - Neil Gaiman