I use ffmpeg, its the smoothest and most reliable out of all of them. But you have to get dirty with the command line in order to use it (since it has no GUI that I know of), and on top of that, since it grabs an area of the screen you may need some knowledge in video editing to crop out the game area.
One of my old videos demonstrating the results in 720P HD, SM 4.0 and it causes minimal lag.