Out of the 3/4 who have mainly played over Christmas and since Lucas has come back I'd have to say Henderson, who tbf has done a very good job. Since Lucas has come back Gerrard has continued to stay deep and ping his Hollywood balls all over the place, whilst Henderson and Allen have had more freedom (Allen has looked 40 gabillion times better when he hasn't played the holding role).
Sahin did seem to get worse as time went on though, even his passing was very poor and he just looked slower mentally and physically; though he also says he prefers a different role to the one he was playing, which begs the question why he wasn't used in Allen's position before Lucas returned. But I'm surprised we haven't used him a little more than we have which is disappointing as I still think he is a good player and he did show us that in glimpses, not least his awareness and composure on the ball.