This is a very tough choice. I haven't played premades much but Mitchell, Gavin and Zoe from the Roomies seem like they'd be nice people to meet up with.
Regarding my own Sims... Leoni would be great fun (she's a goofball comedian) but there is a slight possibility that my food would be laced with mind or physiology altering potions, so maybe not.
Her husband, Norrell, is one of my favourites to play, but he is essentially a self-absorbed, borderline insane at times, immature, MAJOR diva who has a habit of causing arguements, so probably not.
I think my dream lunch would be a beach BBQ on Mua Pel'am with my writer couple, Tadeo and Andromeda, plus the island's residents, Ikkanu (mermaid), Kio (good vampire), and Samphy (Spellcaster).
We could all talk about what being an occult is like. Kio could provide the food (he is an excellent cook).
Tadeo, with his knowledge of ancient languages, could translate what Ikkanu is saying (Ikkanu does speak a bit of English/Simlish but not confidently around strangers).
I'd also enjoy talking creative writing with Tadeo and Andromeda.
Samphy and Andromeda would also help get conversation going, as Tadeo, Kio and Ikkanu are all quite shy when they first meet people.
There's a high possiblity that Norrell and Leoni would crash the party anyway and prank everyone. ?