Artificial ones. I'm not saying that just to be silly. I like the fake one I have because it doesn't drop needles or dry out to become a fire hazard. Also, you can't bend the branches of a real tree and fill in the gaps, but you can with an artificial one. When I was a kid, we always had a blue spruce and it seemed to retain needles well and the branches were far enough apart we could put tons of decorations on it.