Your word, honorificabilitudinitatibus, seems to be commonly regarded as the longest, and it has the distinction of being used by Shakespeare.
However, there's also this one with 28 letters:
which is the action or habit of estimating something as worthless.
Although the latter is longer by one letter, it was coined by students at Eton College as a humorous word. It can be broken down as follows:
floccus (“a wisp”) + naucum (“a trifle”) + nihilum (“nothing”) + pilus (“a hair”) + ficatio
Given that floccinaucinihilipilificatio is of late coinage, I believe that honorificabilitudinitatibus is most likely the winner. Besides being used by Shakespeare, it wasn't coined by him. Rather, it appeared long before his day, such as in the following dictionary entry by Johannes Balbus in 1286: