This is so hard to pick. There are 2 clear things contending for the prize of favourite, so I think I’ll just have to cave and call it a draw. It’s probably telling about me that both are series of books.
First up is Neal Asher’s Polity series of books. Set in the future of humanity where AIs are our benevolent rulers, alien life exists (but there seem to be virtually no intelligent ones) and people can back up their minds, run awesome levels of cybernetics and biological augmentations and travel the galaxy near instantly through a travel system called runcibles that portal through “null-space”. It’s high-action, high-tech sci-fi and it’s awesome. Gridlinked was my first exposure to the series; I’ve loved every second of it and I’ve brought every book since.
I did once attempt to run some games back in my teens using a custom hack I made of the West End Games D6 system. It was fun, but didn’t last very long for one reason or another I forget.
Next is China Miéville’s Bas-Lag setting from his books Perdito Street Station, The Scar and others. The beetle-headed Khepri, the water-like Vodyanoi, the tortured Remade, sentient Cacti… the warping magical storms of the Torque. So much awesome packed into one place, the setting is stunning in both richness and depth. I dearly love pretty much everything about the books set in the world of Bas-Lag and the city of New Crobuzon and to be able to enter that world myself as a player, or craft my own stories there for others as a GM would be a dream come true.
I’ve heard rumours a game was in the works by someone, or at least someone had secured the licensing for making one, but I’ve never heard much else beyond that.