Some would say NOON, but that's only if its written in all uppercase. (noon would be something like uoou upside down.) The word that would best fit would be I as it is always written in uppercase.
tot
NOON
the same way upside down
noon
this \
NOON
Yes, you can flip it upside down to get a d, you can flip it backwards to get a p and u can flip it upside down and backwards to get a b
NOON, SWIMS
SOS
The word, "I." NOON
The word SWIMS, when written in upper case letters, is the longest word in the English language that reads the same both forward and upside-down with no reading difficulty.
backwards or upside down