If your Mid Carry is Good but your ADC is Bad and you are the support.

Biggest Pet Peeve as Support (laning phase is over): "GOD DAMNIT SUPPORT YOU ARE SUPPOSED TO FOLLOW ME ALL OVER THE MAP".

No sir, when lane ends, I am the teams support, not the ADC's personal bodyguard.

The top laner if he does good :] or maybe the jungler who got the mid laner fed in the first place

You support whoever needs it the most. If your mid is good and they're positioned well then you may as well peel for your ADC. Even if they're bad they still need to do damage in fights or you're going to lose the game.

As a support, you should support who is most likely to win you that game. Make the plays that are most likely to win you that fight, that engagement, etc...

If you have a brand who can burst people down and just needs protection during cooldowns, and you have a caitlyn who can never seem to stay away from that fizz and gets deleted every fight; then protect that brand. Or, based on who you are, capture that fish when he goes after your adc. If the person is constantly getting caught your team can at least capitalize on the enemy using ultimates and summoners.

as bad as he/she may be, you will still need to help the adc cause they are usually the biggest tower siege-ers late game. With a few exceptions..like tryn/fiora/yi etc.