1 v 1 Alduin I'd think, maybe. Close fight.

In terms of military power and/or allies, Celestia. It is noted that even the dragon's Alduin raised from the dead aren't all aligned to him and often act for their own selfish reasons.

Well, before he was beaten by the Dragonborn, if he called for the other dragons to fight for him, they would have, since he;s the strongest and their leader.

After he was beaten and fled to Sovengarde, there's a good chance a lot of the dragons wouldn't bother to listen to them if he asked for help.

And if we're going with full military power/allies, Alduin also has all those Draugr and Dragon Priests from the Dragon Cult on his side.

not sure how well ponies would handle Undead, though Spike's comments make it clear they are at least aware of, if only in fiction, zombies.

Should be noted Celestia likely has more than just Spike on her side when it comes to dragons too. Which, if we're crossing the universes a bit for this fight, likely means Thu'um users too. Though that might be assuming too much and is only speculation.

I can't say for certain what all she does have, BUT her confirmed forces include: Royal Guard (along with the implied army behind it), Shining Armor, Princess Cadence, Princess Luna, the Elements of Harmony, alliance with the Saddle Arabians and lest we forget, the recently reformed reality warper Discord.

Me

Bitch, you couldn't even defeat Mirmulnir.

Yeah, he couldn't, that pussy Dovahkiin had help. Id like to see him fight me one on one.

I am Dragonborn

Wouldn't Princess Celestia automatically win? Isn't she like, god?

And Alduin is either the first son or an aspect of Akatosh.

So a dragon is the child of some other dragon?

I don't see your point.

I'll explain, since you aren't well-versed in Elder Scrolls lore.

Akatosh is not just a dragon. Akatosh is an Aedra. He is the Dragon God of Time. The first God to form. The leader of the Nine Dvines.

And Alduin is his firstborn and most powerful child.

And the ''dragon born'' (a.k.a the man that can shout really loud) is that amazing that he can actually slay this dragon?

Yet this dragon born can die by almost any aggressive A.I?

Yeah I think I am sticking with the fucking creator of everything.

It has never been stated that Celestia did anything of the sort.

And the Dragonborn was only able to defeat Alduin because of a prophecy and Dragonrend.

A show and a game, both hilariously flawed when it comes to it's story.

This is why real people don't enjoy neither.

I think he was the first "child" of Akatosh.

So do I, but it was God knows how long ago, and it's a bit fuzzy.