Which is better MAX function or ORDER BY column_name DESC with LIMIT 1
Add an index on created_at and use your first query:

SELECT MAX(created_at) AS latest FROM sample_table;


The suggestion you got is good in other situations, like if you want a max value of a field that is smaller than certain value. i.e. 

SELECT created_at FROM sample_table WHERE created_at&lt;"2014-01-01 00:00:00" ORDER BY created_at DESC LIMIT 1;