From the books and films Rohan is my absolute favorite Good force (with the Ents in #2…in the films Ents are nearly equal because those scenes are just so awesome).
In the games, I love Rohan the most for Good, but have only played them in SBG with borrowed forces. I have been collecting them since day 1 and have in my backlog just about every Hero, multiple Captains, and enough models for 24 Companies of RoR, 4 Companies Royal Knights, and 8 Companies of WoR. But I’ve remained gun-shy about painting them until I felt I could do justice to them because I hold them in such high regard.
My favorite and by far largest Good army in play are Thranduil’s best. A very fragile force but with a lot of potential. They require constant attention and forethought to play competitively, they look good both in small and large numbers and have a lot of options open to them. They ally well with Ents which is a plus.
For Darkness, in the books and films I liked Saruman’s forces the most. Something about one of the greatest of wizards, corrupted, held up in an ancient tower, aspiring to become a Power and building a force to take over the world. I don’t know if I’d say “inspiring” but…well…it is a career path that has it’s attractions.
PJ increased his role well in all areas except I miss his downfall from the books. The Haradrim were a close second in the books and a very close second in the films.
In the games, Isengard was my first Evil army and was really a good learning force. Very forgiving when you’re new and very strong when you are getting better. But I lost some interest in the heavily armored Uruks because they didn’t really challenge me much any more. I still enjoy the Uruk Scouts with Orcs as they have as much potential but require a little more attention during play.
Harad and Dol Guldur have become my favorite Evil armies over the last couple years though and are probably where my love is going to remain for some time to come.
But my absolute favorite force of Evil from the books and films are the Nazgul. Not the cheezy named things that GW gave us, but the original nameless Black Riders. I will never field a named Nazgul in SBG except the WK and Khamul (but even then only with an Easterling or Dol Guldur army). In WotR there is no such choice, curses be to GW on that.