One of the hardest things to do as a DM is to make the transition from an awesome idea to an awesome game. It happens to all of us: We have a moment of clarity and inspiration in which an absolutely amazing combat unfolds in our imagination, we put it together, and then, somehow, it falls flat. This is alright with a normal encounter, when all that's on the line is a couple generic goblins, but at the climax of an adventure, you want to do everything you can to make sure it's memorable.

1. Think 'Story,' Not 'Fight': A good encounter (combat or otherwise) unfolds like a good story, with a distinct beginning, middle, and end. For example, the climactic battle against a goblin horde might begin with a swarm of goblin barbarians to soften the party up and draw out some of their spells and limited-use abilities, as well as hold them back as, in a nearby chamber, goblin shamans perform a summoning ritual. The shamans complete the ritual as the PCs break through the barbarians, and magical energy gathers over their summoning circle as the party takes on the shamans and their guards; finally, after a couple rounds, a demon emerges from the circle and attacks everyone, goblins and PCs alike.
2. Make It Yours: If you're new or don't have the time, it's perfectly fine to use a monster or NPC straight out of a book, but the best encounters are usually the ones you build from the ground up yourself. For an encounter to run smoothly, you need to know the ins and outs of each participant, and when you build an NPC yourself you know exactly what it's capable of. With monsters it's a different issue entirely: Many players out there are very familiar with many standard monsters, which is okay sometimes, but for a climactic battle the mystery of a monster with unknown abilities can really enhance the emotional intensity of the battle. Even just writing up a unique signature spell for a your evil wizard or adding claw attacks to your vampire can change things up enough to make a fight memorable.
3. Have A Plan: Over the years, I've scaled back a lot in terms of what I plan out beforehand for my adventures, but climactic encounters are one of the things I always spend prep time on. After you have an idea how the battle is going to go and have your NPCs and monsters statted up, you still have to make sure things are interesting from a mechanical perspective as well. After all, things are going to get boring fast if all your PCs have to do on their turns is 5-foot step and full attack. Draw up a map of the battlefield and add at least one interesting terrain feature. This can be something as simple as the tried and true bridge over river of lava, or something more unusual, like a swirling cloud of souls that has the ability to temporarily possess those who enter. Your plan should account for how the NPCs and monsters deal with the terrain and how they guide the PCs through it as well.

By following these rules, you should vastly increase the chances that your climactic battles are remembered. Of course, there's always the chance that one of your key NPCs gets crit'd right off the bat or the encounter overwhelms the PCs (all the normal stuff), but remember, it's just a game, so don't panic. And, as the DM, you always get to fight another day.

So here's my question to you, Rockers: What's the most memorable encounter you've ever played in?