I'm a little hung up on semantics with this one. Yesterday's question asked about "the crunchiest RPG" I've ever played, which I took to mean the most rules-heavy. But I don't necessarily equate "fluff" with being the opposite, as in rules-lite. To me, the term "fluff" (which I really, really am not a fan of, incidentally) denotes setting details. So a really "fluffy" game would be one, by my understanding, that has a metric tonne of back story. Something like Harn leaps to mind, although I've never played it.
|Comic by Ryan Pequin.|
But really, you can't beat good old Planet Earth for maximum fluff-factor. In fact, it's my favorite "game world" - almost all of my gaming these days takes place on some version of our Earth, even if that version happens to feature faeries or shoggoths or mad scientists or rotting Midgaard Serpent carcasses or whatever. Three-time soapbox derby champion Ken Hite has laid out the case for choosing Earth over a fictive world nine times out of ten much more effectively than I ever could. Owing to the fact that pretty much everything I've run in recent years has been "enjoyable," I'd say this approach has worked out really well for me. The one downside, really, is that I have a feeling it will take me a lifetime to even begin to absorb all the information in the setting guide.