Most people say 1" OD tubing but I had some 1 1/4" OD that worked just fine with a snug fit. Regular old vinyl tubing. Some people use a smaller diameter tubing (~1/2" ?) and connect it to the tube inside a 3 piece airlock. I personally like using the big tube stuck in the neck of the carboy method myself.....less to clean.
Primary - Barleywine, APA, IPA, Strawberry, Pumpkin
Kegged - Raspberry Wheat
Bottled - Oatmeal Stout
On Deck -