No spacer is best, you can fine tune engines slightly with a spacer but this is a bandaid fix usually. Modern intakes are so good nowadays there usually is no reason to use a spacer unless you need to get a pcv in there. Those spacers you see on the shelf have been there since the early sixties and have not changed to match modern intakes. I would use the shortest spacer you can find to get a pcv on your engine and none if you don't need it.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo