All of them are pretty much equal importance, besides one - Support.
Now before you start hating, read this:
If mid, adc, jungle, or top, do extremely well, they can bring the following to their team.
Mid: Tremendous damage/CC if mid laner has CC (since they will get CDR if they have enough gold)
ADC: Massive damage and clean-up potential.
Jungle: Varies on the jungler, but mostly: Damage, CC, initiation, dueling, mobility.
Top: In order for this to work, they have to build damage. A fed tank is not very useful without damage. So...: Damage, CC (hopefully), and initiation.
Now... the Support: Healing (maybe), CC, and sometimes initiation.
Why Support ranks the lowest, is this: No damage. It's also the same reason you never build full tank when ahead. If your team does not have enough damage/follow up, you will lose. Simple as that. Especially because building Cdr doesn't increase your CC's effectiveness, but just how often it's usable. Supports will rarely ever bring damage (unless it's some Annie or something), and therefore your team will be out stat-ed, and killed.
Of course, if the Support is doing well, the ADC is most likely also doing well. However, assuming they aren't, they will still be useless.
Another reason is that if a Support is forced into a 1v1 situation, they will lose, because they don't build damage. A feeder will likely kill them, or they'll just get away. With anyone else, you could turn it around and earn a kill for your team.
Of course, there's obvious supports who will always drastically affect the game, such as Blitzcrank, Thresh, and Alistar, to name a few. However, most of them will do their job whether or not they're fed. Unless they're feeding, and the reason they're feeding is because they're bad. Then they won't accomplish anything, and it's hard to imagine a scenario where they would get fed.
If there's anything wrong with my reasoning, please let me know.
That being said, I can't really vote unless you allow multiple votes.