So when I saw this topic I got a little excited. Tons of anime have really epic scenes, how can I pick just one? With that in mind, to narrow down the list, I tried to define "epic". I came up with a few criteria:
1. It must be of paramount importance to the story. I'm not talking about some random epic things that a character has done, it has to mean something.
2. It must involve some incredible talent, effort, and/or dedication. I'm not letting these characters slack off and get easy epic moments.
3. The scene must have epic music. Anime music is some of my favourite, and the feel of a truly epic scene will have truly epic music.
So I came up with two scenes. The two fit both criteria quite nicely, but one just barely edges out the other. In second place, we have one of the most epic moments from one of my favourite epic anime
Simon's rebirth scene from Tengen Toppa Gurren Lagann. This is, of course, really important to the story and Simon's character development, as it allows him to move past a jarring death of a loved one. In this scene, Simon goes from wimpy to badass in 2 minutes flat. No ifs, ands, or buts about it, Simon becomes awesome. Not only that, but it ends with one of the most iconic attacks in all of anime, the Giga Drill Breaker. Truly a great scene.
However, this next scene beats it in all those respects, in my opinion. It is the pinnacle of what I would call absolutely epic scenes. In context, it is jawdroppingly awesome. The winner here is
Lelouch's proclamation from Code Geass. This scene is, in my opinion, the most important scene in all of Code Geass, a show ripe with epic scenes and amazing plot development. In this scene Lelouch has reached the culmination of his efforts, and declares complete victory over his opponents. In this case, that would be the entire world. If Lelouch failed here, he would not be able to complete his true motive. This took all of his power to achieve - this is the last episode of the series, and halfway through it at that. He spent literally two seasons for exactly this, and that makes it epic.
Not only that, but the music absolutely perfect for the scene. The fanfare sounds just dark enough to make it seem a victory for the who the world thinks is the bad guys, yet it still sounds insurmountable. It perfectly describes complete victory with no chance of recovery. With all that in mind, Lelouch's proclamation is the number one epic scene I have witnessed to date.