For me, it varies a lot, but whoever starts near salt has an increased chance of winning the game. If there ever is an incoming patch, I would not be surprised if salt is nerfed or tweeked (-1 food, maybe +1 gold). Next best luxury resource is citrus combined with bananas for a jungle start, giving you +3 food immediately. So I am stuck with my choice. Also the location is highly dependent on type of map and civ. If you are Venice, Netherlands or Indonesia, coastal starts are a must.
So I may as well do process of elimination: tundra is literally the worst starting spot, then plains, then jungle, then coast, then desert, then grassland. So by default, I guess I will vote grassland. The poll also fails to mention hills, rivers and mountains. Hills offer improved defences and production, rivers improve trade revenue and mountains unlock the mountain wonders and observatories, which are basically NC's.
EDIT: To David, welcome to CFC. Salt is mentioned a lot because it is an insanely OP luxury resource. Unworked, it gives 3 food on grasslands (2 food on plains), along with +1 production and +1 gold. When salt is improved, it offers 4 food, 2 production and 2 gold (with some variation depending on terrain).