M.C. site of CA oesophagus is -
Esophageal Carcinoma is of two common histological types


Squamous cell Ca - the MC type in the world (-95% according to Sabiston)
Adenocarcinoma - where incidence is increasing at a rapid rate and is now the MC type in the USA (Ref:• Harrison, Schwartz)


Distribution of Squamous Cell Ca.

Upper 1/3 → 10%
Middle 1/3 → 60%
Lower 1/3 → 30%     

Adenocarcinoma is mainly located in lower 1/3.
Nowhere, I could get the MC site of esophageal carcinoma overall (including both squamous and adeno variety). 
But one thing is sure - squamous cell Ca is the MC type of esophageal cancer in the world 


So I presume that the overall MC site would be the MC site involved by the MC type of esophageal cancer i.e. → Middle 1/3

So remember
MC type of esophageal Ca in world → Squamous Cell Ca (-95%) MC site → Middle 1/3
Middle 1/3rd
Upper 1/3rd
Lower 1/3rd
Lower end of esophagus