The Sky and [some] Sea gods are usually a good time, but those aside I’d probably say Vrtra was always my favorite HNM to fight in retail, and Stroper Chyme my favorite regular NM... I remember the fun days of going /BST and charming it and teasing all the gilsellers with him.
On Nasomi, I still love the OWNM fights.. they’re good fun (especially the lower-level fights - much more challenging IMO). To be honest, I wish he’d add even more custom fights. They don’t even need to have phat loot, just something else that’s fun to fight

Maybe something like force-popped ground Kings with no rage timer, 5x the HP, hit 2-3x harder and have a 20% haste? Could offer up silly titles and and mog furniture as the reward and I’m sure people would still be all over them.