Assuming you mean as an opponent to the party, there are many that I am partial to, but my favorite would have to be Dragons. That said, in over 23 years of DM'ing, I have only run less than a dozen battles against dragons that I can remember, but to me they are D&D's ultimate iconic creature(s). I mean, it is called Dungeons & DRAGONS for a reason. Imagine Dungeons & Illithids or Dungeons & Githyanki or some such nonsense. Bah! Give me a Dragon anyday.
DM
(a little tongue in cheek, but mostly serious)