It's not
really what you're asking, but my pick would be the countdown collection pair of
0P42 and
0P36, best shown here:
https://wiki.lotrtcgpc.net/wiki/The_Countdown_Collection.
More conventionally, there are some card images I've thought are cool enough to force into a deck regardless of whether I really
should be using them. The one that comes to mind first is
Above the Battlement, as I've actually grown quite fond of its text too and put it in most of my Sauron decks now. The red-and-black is perfect for a Sauron card.
I also want to throw out The
Citadel of Minas Tirith. This was the favorite card of the guy who introduced me to the game. In his words: good title, good picture, good text, good lore.
Lastly, shout-out to
the Radioactive Pig.