* Longest word: "Floccinaucinihilipilification" (29 letters) is a word meaning "the act of estimating something as worthless". It's hard to remember and spell due to its length, but not necessarily difficult in terms of spelling rules.
* Unusual spelling: Words like "pneumonoultramicroscopicsilicovolcanoconiosis" (45 letters) are difficult because they have a lot of unusual letters and combinations. This word refers to a lung disease caused by inhaling very fine silica dust.
* Tricky rules: Some words like "their" and "there" are difficult because they sound the same but have different spellings.
Ultimately, the hardest word to spell is the one you personally struggle with most!