Keying out greenscreen is tricky business. There are so many factors to doing this well. Making sure that you light the subject to match the lighting of the eventual background, perspective, camera angles for both shots, and this is all outside of the software.
Adobe After Effects has one of the most versatile and easy to use greenscreen keying. It's labeled only "keying" in the plug ins, and there are several variations. You can do a color range, or just a single color with some degree of tolerance levels to control.
YouTube has a plethra of free tutorials on basics of greenscreen.