318
Pretty good. It's what I have in the Demon. I rebuilt the 68 318 with 302 heads, Performer, AFB and a 276/286 .441 lift cam. Mine's a driver and I'm happy with it. If you're after more power, certainly a 340, 360, or BB is the way to go.
I actually prefer the 67-69 318s because they have the highest stock compression ratio of any of them. They don't have the durability legend of the /6, but it doesn't miss by much.