Everybody has favorites, but next to the Remington factory model (which sells for around $80.00) the best buy for the quality/price is the Choate.
The Choate comes as a complete kit with the extension in satin blue or parkerized, in any length from +2 to +10, and with a longer spring, a heavy-duty magazine follower, and the barrel clamp and sling swivel.
For a 20" barrel, you'll need a +3 extension.
The Choate has been around almost as long as the original Remington factory version and I installed them for 30 years with only one or two problems due to a factory defect.
I'd buy from Brownell's because they not only stand behind what they sell 100%, they'll also send you a free copy of their catalog, which is the best "gun stuff" catalog in the world.