In Astral Chain all three Jena Anderson themes and the Noah and Noah Prime themes. Excellent stuff.

Of course, this is difficult not to go for a recency bias judgement on this question. Too many incredible battle themes have graced my mind during the time I've been gaming ...

I'll come back to edit the post after I decided on what to put on here

The Final Fantasy series usually have very good battle/boss themes.

I would post a lot of them, so it's easier to just mention the whole franchise.

Unironically, this is in my Top 10.

I think the OG Halo trilogy battle music was pretty epic. If you know the franchise, you'll likely start hearing it even before you finish this sentence.

There are many to name but these were the first ones that came to mind and they're surely in my top 5 or so.

I'm impressed indeed.

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Thank you to all, I only knew 1/3 of the one you posted. I forgot to say that you can post as many themes as you want. 

Like Alex said there are so many Final Fantasy's themes to put that I'm lazy to do it. ^^

Other than the entire Furi soundtrack.

From KH2:

This literal masterwork from Darkest Dungeon:

More recently (build up and melody are amazing, far better than main boss theme from the game):

And a classic from a fighting game: