@Scrounge  
Except for diversity of magical abilities, as changelings have about as many spells outside their signature shapeshifting as an uneducated unicorn.  
It would be fascinating to discover whether they can use pony spells if they learn them.  
If so, then the potential is there and Neighsay has much to fear

@Stotter  
By those standards, changelings might be more “ultimate” than alicorns.

@Scrounge  
That pretty hair can apply to Ultimate Kars and alicorns amuses me.
 
To answer the question though, I would have to say a mix of fast adaptability, diversity of habitable biomes, and diversity of abilities. I feel like that last part would let something besides bacteria claim the title.

@Stotter
 
Depends on how you define “ultimate”. Raw power? Capacity for survival? Pretty hair?

@Ryodraco  
The alicorns having all their traits in one, they are the ultimate life form  
(Avalon plays)

@Stotter  
Plus their bodies seem to be pretty tough (they often take major falls and impacts with little or no injury, and not always for the sake of comedy).

@JP  
Maybe ponies have magic because of the evolutionary pressure of having so many predators to fend off.

@PonyPon  
Oh, no, there’s plenty of nasty stuff stuck at ground level, too. Hydras, the chimera, tatzlwurms, cragadiles, maulwurfs, ursas, pukwudgies…
 
Equestria’s got a whole bunch of different monster things.
 
 
@Ardashir  
Despite the wide variety of monster species, none of them actually seem to be particularly common. Most only turn up once, and I think only ones who got used in word-building (basically windigos and timber wolves) showed more than twice.

It seems all their worst stuff is capable of flight.

At least ponies have magic to defend themselves with.
 
We haven’t seen magic-casting flying and biting deaththings yet.

@Scrounge  
The flying bear that stings like a hornet, you mean?
 
How could I forget that? No, really, how, because I need to forget it again. shivers in fear

@Background Pony Number 17  
You forgot the flying bear.
 

@Background Pony Number 17  

Still slightly better than Australia though.

@laofuzi  
It was. I misremembered.

I’m beginning to wonder how anyone lives to adulthood in Equestria. Australia looks more hospitable.

@MezzoDragon  
I thought that was a crocodile!

@laofuzi  
No. Just alligators made of rock.

@Background Pony Number 17  
What’s next, flying alligators??

First flying spiders, now flying piranha. They’re going out of their way to make Equestria look like the most inhospitable place of the face of the Earth.

@Background Pony #CA40  
Whatever you say, you started the argument in the first place.

@Background Pony #0A20  
Yeah you did

@Background Pony #CA40  
I just like piranhas, I haven’t seen the episode.

@Background Pony #0A20  
Liar

@Background Pony #CA40  
Nah

@Background Pony #0A20  
Their the worst