As a Gm of 26 years standing, I can honestly say that the best games I have run have been those where I just did it off the top of my head, no prep or scenario to work from. The wingit stuff always seems to produce memorable games. Mind you, helps to know the world and system inside out - I sometimes feel like I live in Glorantha
Games with hundreds of books and charts always seem to leave the players bored and half the night is wasted thumbing through supliments. A case in point was cyberpunk games. Ran FASA's Shadowrun and it was awful, despite the great setting and some nice ideas. Ran virtually the same stuff with the Cyberpunk game which kept everything simple, and it was excellent. Another was C&S Vs Pendragon. Playing C&S was like attending a very boring lecture on medieval microsystems and the much simpler pendragon was a blast.