These phrases are not correct or incorrect, they're expressions. Which you use would depend on what the people around you use. Someone I used to work with would often say, "by the by and by".
Correct answer typically implies there is one answer to the question. However, best answer can also be used if multiple answers COULD be correct, but one is better than the others.
"Stationery" is one of two words with the same pronunciation, but different spellings and meanings. "Stationery" means writing paper and envelopes; "stationary" means not moving.