best cantonese seafood restaurant in greater LA area.
Really depends on what type of dishes you are looking for and exactly what you mean by "cantonese".
That being said, I really like SEA HARBOUR on Rosemead Blvd. (in Rosemead) for solid and inventive seafood offerings. Make sure to inquire about off-menu, daily market specials.
Another good option is SEAFOOD VILLAGE (with 2 locations, Temple City and Monterey Park), that offers really good chiu-chou style seafood dishes.