Wow, way too many to choose from!
I think my favorite right now is Isaiah 58.
3 'Why have we fasted,' they say,
'and you have not seen it?
Why have we humbled ourselves,
and you have not noticed?'
"Yet on the day of your fasting, you do as you please
and exploit all your workers.
4 Your fasting ends in quarreling and strife,
and in striking each other with wicked fists.
You cannot fast as you do today
and expect your voice to be heard on high.
5 Is this the kind of fast I have chosen,
only a day for a man to humble himself?
Is it only for bowing one's head like a reed
and for lying on sackcloth and ashes?
Is that what you call a fast,
a day acceptable to the LORD ?
6 "Is not this the kind of fasting I have chosen:
to loose the chains of injustice
and untie the cords of the yoke,
to set the oppressed free
and break every yoke?
7 Is it not to share your food with the hungry
and to provide the poor wanderer with shelter—
when you see the naked, to clothe him,
and not to turn away from your own flesh and blood?
8 Then your light will break forth like the dawn,
and your healing will quickly appear;
then your righteousness [a] will go before you,
and the glory of the LORD will be your rear guard.
9 Then you will call, and the LORD will answer;
you will cry for help, and he will say: Here am I.
"If you do away with the yoke of oppression,
with the pointing finger and malicious talk,
10 and if you spend yourselves in behalf of the hungry
and satisfy the needs of the oppressed,
then your light will rise in the darkness,
and your night will become like the noonday.
"For the High and Exalted One who lives forever, whose name is Holy says this: 'I live in a high and holy place, and with the oppressed and lowly of spirit, to revive the spirit of the lowly and revive the heart of the oppressed." - Isaiah57:15