theres the choices of the 4 cults which imo are the best
(nurgle, tzeentch, slaanesh and khorne)
tzeentch seem to me more of firepower army..
khorne seem to be a full on close combat army..
nurgle for me is a mix of both but these are really tough..
and slaanesh im not to sure bout but they have some good rules modles e.t.c plus lucious the eternal is really good
then theres the traitor legions,
knightlords, black legion, iron warriors, blah blah blah....
if i were you id read up about them see which one you feel like readiong more.. then look through the models and as a painter and a gamer which one appeals to you most.. then it goes from there