The Fiend Folio was my very first 1st Edition AD&D book. I had started with the classic BECMI D&D books and moved on to the Jeff Easley covered 2nd Edition books. 1st Edition had a certain cache with me, due in part to age, but more to its weirdness. When the guy in my video editing class, his name lost to the ravages of memory, sold me his Fiend Folio for the princely sum of $5, I happily skipped lunch and read it under a tree.
The first thing I noticed about the Fiend Folio was the cover, a bright, strange painting of a githyanki swinging his sword. The early 1st Edition books, with their color covers that spilled onto the spine, were notably different from the late 1st Edition books with orange spines. The strange, arcane art of the cover continued on the inside. Each and every page was illustrated with simple line drawings of the crazy monsters within. Unlike the monsters of D&D, the Fiend Folio presented AD&D monsters that were shockingly alien.
The best part of using the Fiend Folio with 2nd Edition AD&D was that the players had no knowledge of them. Making the players deduce how to defeat monsters in character instead of through out of character memorization makes for memorable and exciting games. Was the Fiend Folio a perfect monster catalog? Nope. There were some nearly useless monsters and an excess of unneeded variants. How many giants does your campaign need, after all?
Ultimately, outside of a 1st Edition, Adventures Dark and Deep, or OSRIC campaign, I have a hard time recommending the Fiend Folio to modern D&D players, which is why it placed so low in my top 5. If you are playing a deeply old school campaign and desire some strange new monsters to mess with your players, the Fiend Folio is a fairly common book and is easy to find in decent condition. Those TSR hardcovers were made to last. Just be warned, finding one that is completely free of highlighter colored monsters and Doritos orange stains can take some doing.