It's a toss up between Pokken Tournament and Splatoon for me. Pokken because of the quick matchmaking, zero lag and disconnects and the depth of gameplay. Splatoon because you can literally spend hours just playing match after match and have incredible fun without being affected so much by lag or disconnects. Though there have been occasional matches where lag was a major problem and disconnections for no apparent reason.
MK8 and Smash have good online multiplayer too but I think Pokken and Splatoon just simply do it better.








