In no particular order, 
Goblin Armory (foil), 
Simbelmyne (foil), 
Terrible and Evil (foil), 
Hides (foil), and 
Hate (foil) strike me as among the top five hardest to get. Mostly because they appear in nearly every version of their deck type either in quantity (i.e., four 
Goblin Armory in a 
![Moria [Moria]](https://lotrtcgdb.com/forums/Smileys/classic/moria.png)
 deck, four 
Hides in a 
![Dunland [Dunland]](https://lotrtcgdb.com/forums/Smileys/classic/dunland.png)
 deck) or in importance (a few copies of 
Simbelmyne appear in most 
![Rohan [Rohan]](https://lotrtcgdb.com/forums/Smileys/classic/rohan.png)
 decks, 
Terrible and Evil is a staple in a 6+ companion deck, and 
Hate is a common 
![Sauron [Sauron]](https://lotrtcgdb.com/forums/Smileys/classic/sauron.png)
 card).
I have seen foil versions of these cards go for between $10 and $25 USD ($25 being the outlier high price). About a year ago I paid $13 for a 
Goblin Armory (foil) and I was more than content with that. But I've been looking for two 
Goblin Armory foil and three 
Hate foils since Fellowship of the Ring came out.
-wtk
Edit: That wasn't meant to be an end-all, be-all list. But those are cards I would likely value near the top of a list. Again, want to remind everyone that I didn't put them in a particular order.
-wtk