Army of the Dead is a popular choice ...
You can pick up the models cheap with the plastic troops, a bargain if you get them in the Pelennor fields box set with lots of other models (or on their own). The riders of the dead, admittedly, are still metal, available from GW website.
The new King of the Dead model is looking very nice.
If you have the main LOTR book, you have the rules to ally the Army of the Dead with eg. Minas Tirith, which means you could put them with Aragorn the King and Minas Tirith troops. (Theoretically, you can ally with any good army, but only certain combinations get you all the bonuses).
But what a lot of people like to do is combine them with Aragorn/Legolas/Gimli from the new "three hunters" plastic set. There are special rules for this combination in the Gondor at War rule-book supplement as a "legendary legion."
Army of the dead, of course, have the advantage of being able to attack popular armies like heavily-armoured dwarves, because they attack vs courage rather than defence.
The army has done well at tournaments. Amusing account here:
https://www.warhammer-community.com/201 ... ge-post-2/