word2vec - what is best? add, concatenate or average word vectors?

I have found an answer in the Stanford lecture "Deep Learning for Natural Language Processing" (Lecture 2, March 2016). It's available here. In minute 46 Richard Socher states that the common way is to average the two word vectors.

You should read this research work at-least once to get the whole idea of combining word embeddings using different algebraic operators. It was my research.

In this paper you can also see the other methods to combine word vectors.

In short L1-Normalized average word vectors and sum of words are good representations.