No adventure author or GM can really predict what their players are going to truly love and remember about any particular session or campaign (in my experience, it’s often to be a groan-worthy joke as anything else*), but you can get a sense of what monsters they’ll remember fighting. If you plan out encounters with these monsters carefully, you can craft encounters they’ll remember long after the campaign is done.

Players’ stories about satisfying, memorable victories against monsters come down to variations on the following:

Power: The monster is widely regarded as being among the most powerful in the game (such as the tarrasque, a beholder, or a demigod), but the PCs defeat them in combat. These victories almost always have entire adventures, and possibly campaigns, leading up to them, so you’ll know well in advance if one of these is on the horizon.

Recurrence: The monster frustrated the PCs for a time leading up to a final fight, so the victory was particularly rewarding (arranging the final defeat of a vampire, lich, or ghost that kept rejuvenating to beleaguer the PCs again and again, for example). A victory over a foe that the PCs “love to hate” like this can be rewarding, but risks being frustrating in the early encounters when the PCs can’t seem to win no matter how hard they try. Two and-then-they-got-away encounters with this kind of foe is usually enough to set him or her up as a recurring villain; more than that starts to feel like a grind and loses impact.

Heroic Impact: The PCs heard a lot about the monster from NPCs (particularly about its invulnerability, great devastation, or other similarly impressive features) and went on to confront and defeat it anyway. PCs tend to remember this not for the challenge of the monster itself—although they might—but from the NPCs. “And we saved the whole town!” is only a memorable result if the town itself was memorable, and the PCs had connections to the NPCs within it. Both NPC reactions to the danger and accolades to the PCs afterwards help build this in the players’ minds.

Incremental Work: The PCs worked hard for the victory. PCs that need to engage in several mini-missions to collect magical components, weaken a villain’s energies, or similar tasks often find their ultimate victory particularly rewarding. The risk is that these preliminary missions seems like mere fetch-quests, so each should stand mostly alone as a narrative: a compelling villain, meaningful interactions with NPCs, and results beyond “we got the next item on the list.”

Luck: The PCs defeated a difficult monster due to sheer luck, such as a timely critical hit. These sorts of things can’t often be planned, but remain memorable nonetheless. When they occur, they’re more memorable if they have an in-game reaction: a critical hit that drops the orc king means the entire orc tribe surrenders, or a critical spell roll that banishes a demon prince means other demons swear fealty to the PCs.

* One that I remember from so long ago that I don’t recall anything else from the campaign: my friend’s elf when into a bar in a xenophobic human town, and the bartender said, “hey, we don’t serve your kind,” to which my friend responded without missing a beat, “that’s okay, I don’t eat my kind.”