1/4" Barb to 1/4" female flare fitting. Use on MFL (threaded) corny disconnects with 1/4" ID hose. This is the most common size for corny kegs. 3/16" ID beer line will fit onto 1/4" barb but it is a very tight fit and the hose must be heated under hot tap water first.
Use a swivel nut washer when you have a metal-to-metal contact.