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.
The Kitchen in Alhambra on Valley. Not cheap, but as close to Hong Kong as you'll find in LA. Second would be Triumphal Palace, also in Alhambra on Main.