Everything and Nothing
It all depends on the atmosphere, campaign premise, etc that you're trying to develop and portray. In a campaign themed on a 'realistic' depiction of medieval Europe, almost 90% of D&D is too silly or at the very least inappropriate. Its filled with Rust Monsters, Ropers, Color Cordinated Dragons and all manner of categorized Elves, Dwarves and other assorted species that are broken down by Sun, Moon, Wood, Mountain, Dark and many more. Been wondering what's taking them so long to introduce Cold Bog Elves, Half Moon Dwarves, Whole Halflings and Pumperknickel Gnomes.
At the same time, all those fun things (expect the Pumperknickel Gnomes) and a ton of others are in at least one or more of my campaign worlds. Whatever works I always say.
In addition I love comedic campaigns. Not silly per se but humorous, Sitcom/Dramedy campaigns with great characters, great stories and loads of laughs. Oh it just doesn't get any better!
So go on D&D fans, pile on the Modrons, hoist the Flumphs and ride that crazy hatchbacked Bullete into the sunset!
AD
"Guy, Guy... maybe you're the plucky comic relief. You ever think about that?"