Well, you say it is a riddle, so my answer is that the LONGEST word in the dictionary is
the word "LONGEST".
No other word is actually LONGEST, no matter how many more letters it has.
The Oxford English Dictionary (OED) includes the word:
which, with 45 letters, has more letters than any other word in the OED. But despite that fact even Pneumonoultramicroscopicsilicovolcanoconios is not LONGEST.
I vaguely remember this, though this might be the answer to some other riddle.
"Smiles" there's a mile between each 's'.
Although in hindsight, the plural of a noun,smile, usually is not included in a dictionary.
Also if you mean in any language, then German has some really long ones because they just keep tacking words together to describe new things.
But the answer to that fourth grade riddle is SMILES as Bathory says.
I'm pretty sure it's antidisestablishmentarianism.