The YJ MGC 4x4x4 is the most popular 4x4x4 on speedcubing.org at the moment, however, it is nearly 4 years old now and has recently been surpassed in many ways by the new Vin cube, YJ seem to have found some way of re-releasing the same design but with a unique feature on the spring that significantly alters the feel. Based on my first impressions of it though, it doesn't seem to be helping things very much, it seems to make the cube turn much faster but also be more uncontrollable and lock-up more frequently. You can buy it here