How to list files recursively and sort them by modification time?

Use find's -printf and sort on a reasonable date format:

find -type f -printf '%T+\t%p\n' | sort -n

This should minimize process forks and thus be the fastest.

Examples if you don't like the fractional second part (which is often not implemented in the file system anyway):

find -type f -printf '%T+\t%p\n' | sed 's/\.[[:digit:]]\{10\}//' | sort -n
find -type f -printf '%T+\t%p\n' | cut --complement -c 20-30 | sort -n

EDIT: Standard find on Mac does not have -printf. But it is not difficult to install GNU find on Mac (also see that link for more caveats concerning Mac/Linux compatibility and xargs).

Here is a method using stat as @johnshen64 suggested

find . -type f -exec stat -f "%m%t%Sm %N" '{}' \; | sort -rn | head -20 | cut -f2-

This answer to a similar question on the Unix Stack Exchange site helped me because I was using zsh:

How to list files sorted by modification date recursively (no stat command available!)

find . should be able to get all the files. Something like this:

find . -exec ls -dl '{}' \; | sort -k 6,7

You need to tune it for you needs.