NOD and SCRIN. Out of GDI and NOD, NOD is good to play with, as Ck Nish said, they have weaker units than GDI, but they are fast to build and do tend to be a lot stronger with multiple types of units combined.
But I really liked playing SCRIN as they have more unique units as compared to GDI and NOD. Though, they have weaker ground units, they have awesome air units and as a civ who are more acquainted with Tiberium, their resource gathering rate is a lot faster than both NOD and GDI. So my vote went to SCRIN.