Seriously, if you're not having any fun, why play RPGs?
The most consistently fun RPG I've ever run is Jared Sorensen's Inspectres, the first and arguably still best 'Narrativist' or indie RPG I've encountered. I first downloaded the PDF back in 2005 and since then, I have run dozens of incredibly successful sessions of supernatural hunting fun with gamers and non-gamers alike. Though many games tout their ability to be run relatively prep-free, Inspectres is really the only game that I have been able to run successfully with absolutely NO prep. The player buy-in for Inspectres is honestly stronger than most fantasy or sci-fi RPGs, because EVERYONE has either seen Ghostbusters or watched a Monster of the Week supernatural TV show (Buffy, Angel, X-Files, Supernatural, et al.) Failing that, they can always basically play themselves.
The system is super simple: characters are created with 9 points put into four Attributes (Athletics, Academics, Technology and Contact) and a Talent that adds a bonus to these rolls when in use (ie. Mechanic gets a die bonus when fixing a truck, swinging a monkey wrench, haggling over a van, etc...) Each assignment is measured in Job dice, which are acquired as characters come closer to solving the mystery, defeating the monster or finding the missing MacGuffin. But what makes it so easy to run without any prep is the fact that narrative control is shared (if not dominated) by player input.
When players succeed at their Attribute rolls, they describe the results of their success. When a character is researching the apartment complex inhabited by ghosts and rolls a 6 on one of their Academics dice, they get to decide what is causing the building to be haunted. It could have been built as a natural conductor for ectoplasmic energy like Dana Barrett's apartment building in Ghostbusters or built on top of a textile shop that caught fire, killing everyone within. It's all up to the players, which means that you, as the GM, can just concentrate on setting up the initial scenario and rolling with your player's ideas, keeping the Stress on the whole time.
I've adapted the rules of Inspectres to hardboiled detectives, Leverage-style heists and just about any other episodic genre out there. It's a game that's perfectly designed to emulate that structure and it's up and downs. You can find a copy of the Start-Up rules on the Memento Mori Theatricks website. If you like the idea of the game after reading it, gather up a group of friends and just wing it. It's the best option.
Cover image from the Memento Mori Theatricks website.