In terms of story, can't really name a singular one.
However, in actually fighting against them.....
No two encounters with him were ever really the same. Just with his ability, he changes up his attacks, patterns, health, everything with each incarnation you face. While he'll have some signature moves, you still can't use the same strat you used before. Was always the hardest boss in all of the Battle Network games and he was completely
optional in most cases. Think the only time you're ever forced to fight him is in 3 towards the end.