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Dancing Mad is a final boss battle theme, it's three (or four or five but let's not split hairs too much! 
) tracks in one
) tracks in onewat u did thar
i c it
Anyway, I haven't heard Aeris's theme in-game myself, but even without that, its bittersweet nature is abundantly obvious. The repeated use of C# as a passing-note in the melody during the G (subdominant) chord in the middle/late portion of the track really makes it potently poignant. (If you don't know what I mean, play a G in the left hand on a piano and play B C# D or D C# B in the right hand. And listen to that dissonance between the G and the C#.)
It's for matchups like these that I really wish we could do these by category rather than overall -- Aeris's theme and Dancing Mad just represent two very, very different things, one a boss theme and the other a character/event theme. Comparing them is like comparing the very best in apples to the very best in oranges. The comparison doesn't really make sense.
But anyway, this is what it is.
Dancing Mad's first two tracks sound a little typical as far as major boss themes go -- the first one sounding intimidating, the second sounding psychotically twisted. Then the third track just suddenly comes out of nowhere and sounds beautifully bright and sacred and you start wondering what is going on. You can see the light, too, and there's no way this is the last phase of the battle because it just doesn't make sense that it would be. But if you haven't been spoiled about this game, you won't have had any hint of this surprise, if I recall correctly. That third track track in Dancing Mad is a crucial turning point in the music, and introduces a new element that is visually fulfilled only in the final phase: an unabashed glorification of an agent of nihility, and in C# minor no less, compared to the C# major of the third track.
Oh, and then there's the layering of that chord in fourths, that harks back to the very, very beginning of the game.
And of course, just to put a psychopathic cherry on top, there's Kefka's chilling laugh, burned into the music itself.
A cool feature of Dancing Mad is that there are actually proper outtros for both of the first two tracks. They don't play right on the PC version, apparently, but they DO play right on the SNES version, so here's what you'll get for each:
1. intro
2. loop
3. if you're not done with that phase yet, loop again
4. if you're done with that phase, outtro.
5. next track
I don't think I've ever found such an outtro that's actually used in-game, or at least one I can remember off the top of my head. There's rarely a use for an outtro, granted -- the timing of a track is usually such that when something happens, you don't get a lot of lead time to the next track, so the most you do is a fade out, rather than waiting for the end of the loop. It's just that in FF6's final battle, the stage changes are so lengthy that they can accommodate this, and the result is glorious.
Edit
I thought about this and realized that the outtros could be handled using just offset loops, where the loop contains the main part and the intro, in that order. The second track's loop just doesn't even contain an intro anyway because the intro sounds similar to the beginning of the main part. But still, these first two phases are still distinctive in that they actually let their battle themes play out even after the phases have ended, and the next theme only starts playing when the loop ends, rather than doing a fade-out on the prior track.
I do just wish the final battle was a little more difficult, to really drive this point home. But as a player who didn't bother making any specific character build (e.g. no four-attacks, no dual-wielding, no doublecasting relics, no Ultima) going on, all i basically did was use each character's special abilities repeatedly (with some exceptions obviously) and the fight was still reasonably epic, except maybe the final boss himself going down a little too easily.
In comparison, I haven't seen Aeris's theme in-game. Well, I haven't personally encountered it in-game, though I have watched it, though sadly I never did get the chance to encounter it spoiler-free. It plays during
Possible spoilers: highlight to view
, right? That has got to be a unique feeling. From the perspective of someone who didn't go through the emotional wringer of the whole experience, it seems a bit bizarrely unfitting, but reactions I've read from people who have seem to imply that it's something that is just...pretty unique. "They really know how to upset their audience", in a good way, writes a YouTube commenter named James Fowler.the battle with Jenova*LIFE
I really like both of these options. I really do.
I think I'm going to go with Dancing Mad. It just shows a far greater range of musical mastery than Aeris's theme does. This is, admittedly, due to the fact that it's four tracks in one, which is pretty darn unique on its part, and once this factor is introduced, that pretty much means it wins against anything, so I don't want to use it, but it's hard to find another way to choose.