Since we've been discussing lately games which drove us away due to the great amount of mechanical prep, it seemed like a good time to discuss the games which we can prep stuff for easily.

My king right now is 4th-ED D&D. I'm running a successful game with players up around 4th level, and if I need to create something on-the-fly the DMG has a ton of tools to help out - from a random dungeon generator to room contents to "package" of monsters encountered (wolf pack, dragon's den, double line, etc.) or traps set, etc.

And with 4th, I'm finding it bleedingly easy to "reskin" monsters as often as necessary - a young green dragon became the "Great Old Pumpkin" monster around Hallowe'en, with special effects on its attacks changed as necessary.

So, what games do you find are easy to prep for? Ones you don't spend 45 hours running the numbers for?