The Last Wish raid in Destiny 2:Forsaken immediately comes to my mind. My clan and I always attempted the World’s First run for every new raid, but Last Wish was by far the hardest to complete. It took us a whopping 28 hours to finally beat.
For reference, the clan that beat Last Wish first took nearly 19 hours. The Vault encounter alone nearly made us quit as nearly half of the 28 hours were just for that damn encounter