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.

If you wanna play with your friends – Mario Party is still the safest choice. Mario Kart works most of the time as well, although good players will mostly win the races.

Wii sports games. Had the most Fun with those by far but Mario Kart is so much fun still while those aren't available anymore in their pure form.

The blisters we would get on our hands during Mario Party on the N64... Crazy.