Robert Frost's"Nothing Gold Can Stay" seems like the obvious choice.

1) It is so popular that the majority of people already at least sorta-know it, and a significant percentage could already recite it from memory.

2) It has a cadence that makes it easy to recite in unison.

3) It's simple.

4) It isn't specific to atheism/ doesn't exclude anybody.

5) It faces death rather than rages against it. Funerals probably aren't the best time to rage against death. That's sorta counterproductive to the acceptance that funerals are supposed to provide that helps people move on with their lives in a functioning manner.

Nature’s first green is gold,
Her hardest hue to hold.
Her early leaf’s a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.