This is a tough one. There are a lot of runner-ups: Yggdrasil, The One Ring, Vornheim, Nova Praxis… But if I go with just initial gut-reaction of “cool”, I’d have to say Unhallowed Metropolis.
It’s super dense with tons of information, fluff, moody artwork, enticing photographs, a hip Gas-mask chic thing crossed with goth and steam-punk and… well, character.
It’s a very attractive product. Not a huge fan of the system (a little to crunchy for me on details), but holy hell are there lots of details in this book.
I used it as a setting for a brief (and disastrous) Monster of the Week campaign. Lots of lessons learned, but the biggest was about player expectations about fluff and thematic constraints. My bad.
Some day, I’d really like to run this (albeit with a different game system).
Here is some of that cool artwork :
24th – Most Complicated RPG Owned
25th – Favourite RPG no one else wants to play
26th – Coolest character sheet
27th – Game You’d like to see a new / improved edition of…
28th – Scariest Game you’ve played
29th – Most memorable encounter
30th – Rarest RPG Owned
31st – Favourite RPG of all time