Depends on the group, if it's people used to playing Smash Bros, that's my first pick but otherwise I go for Mariokart since it's accessible to everyone.

Mario party, so much fun

Mario Kart and Super Smash Bros.

More for Smash - even playing the original on N64 with a group of 4 was a blast.

One of my former workplaces had a break room with some game systems - one of them a N64 and it had Smash bros - we would huddle up in the break room during lunch break and play - it was great.

Smash has never had good online. It doesn't seem like Ultimate's online is any better than Wii U's. I suppose using ethernet on Switch OLED's dock or an adapter would improve things, but the connections should be better even with Wi-Fi like most use.
Because of that, Smash. 5-8 Player Smash is a hoot. I remember hours of fun with that in Smash for Wii U.
The Smash games with local multiplayer can't be beat, and I loved playing the Subspace Emissary with my brothers as we tagged in and out with the 2-player limit.