Find every digit from the largest column
Haskell, 63 bytes

import Data.Lists
argmax sum.transpose.map(map(read.pure).show)


Usage example: argmax sum.transpose.map(map(read.pure).show) $ [12,304,506] -> [4,6].

How it works:

                     map                       -- for each number
                         map(read.pure).show   -- turn into list of digits
           transpose                           -- transpose the list of list
argmax sum                                     -- find the element with the
                                               -- largest sum