This is one of my absolute favourite categories of games. So much fun can be packed in without the GM doing any prep work! Here are my suggestions:
Diaspora's first session cluster and character generation is super fun and requires no preparation if you've already read the first few chapters of the rules.
Lady Blackbird is essentially print and play straight away without the GM needing to even read anything ahead of time and it has this magic way of sort of running itself that has to be experienced to be believed.
It Wasn't Me was so much fun! Everyone is a suspect in a murder and over the course of several discussion rounds you determine who did it. As funny or as serious as your group wants. We went the funny route and I laughed so hard! Again requires basically zero reading ahead of time and can be played GMless.
We Are Roommates Now is a silly game where half the players are humans and the other half are their new alien roommates. All the players have to do is construct a roommate agreement in 45 minutes for the benefit of human-alien relations. The entire game is just two sheets of paper, one which tells the humans what they know about the aliens and what they want out of the roommate agreement and another that tells the aliens what they know about humans and what they want out of the roommate agreement. Zero prep and loads of laughs.
Hope one of them catches your eye if you haven't played it
...124 to run fleeing from the mountain. ...125 to use a rope to climb the cliff. ...126 to quickly cast "summon stairs." ...127 to dodge under the falling rocks.
Oh, I have so many! And, the longer I GM some of the bigger systems, the most those fall into the zero-prep end of the scale.
A Doomed Pilgrim in the Ruins of the Future and Atop a Lonely Tower
Cheat Your Own Adventure
The Extraordinary Adventures of Baron Munchausen
The Murder of Mr. Crow
The Quiet Year and The Deep Forest