Glock 21 Holster
One of the good things and bad things about the gun is that it is hot if there is one in the pipe. No extra safeties to flip, no hammer to draw, no clicky sounds to make like in the movies. Pull the trigger and BAM.
If you want to carry in a ready mode it is very important to have a really good holster. Make sure it protects the trigger fully. One stray branch in the woods could ruin your day otherwise.
I have a ITAC Paddle Holster. This holster is very comfortable and the Glock locks into place. Drawing the gun is very easy and natural by depressing the button where my finger naturally rests. It's fully adjustable for cant, that's adjustable with an allen key and has a protective sight channel.
It rides nice and has a matching 2 magazine holder. You can run while wearing it, which is important during a killer robot uprising.
--It won't cost you an arm and a leg either.