The Remington factory extension is very good but expensive and a little hard to get.
For the price/quality, the Choate extension is a "best buy".
It comes as a complete kit with the extension, spring, heavy-duty magazine follower, and a barrel clamp with sling swivels.
It comes in satin blue or parkerized finish and in lengths to fit barrels from 18" to 26".
It's been around for a good many years, and has an excellent reputation for quality.
Buy it from Brownell's, Midway, or direct from Choate.
Tac-Star has at least some history of quality problems so I don't recommend it.
Only the longer versions come with a clamp.
Wilson/Scattergun makes a good extension but doesn't come with a clamp.
Make sure to use a barrel clamp. No matter what someone says about how strong an extension brand is, the weak point is the thin magazine tube on the gun, which is further weakened by the deep threads.
No matter how strong the extension is, if you give the gun a good bump and there's no barrel clamp to support it, the extension can get knocked off the gun, which damages the gun's magazine tube.
That's an expensive trip back to Remington to have a new mag tube brazed in and the gun refinished.
Use a clamp.
