Alright, here’s my personal take on why each timeline is worse than the previous one (Warning, I get too dark):
 
King Sombra’s timeline: There is still a chance of victory but there’s a lot of ponies suffering from this war, this is the least worse of the timelines though.
 
Chrysalis’ timeline: All chances of victory have been lost but there’s still a dying hope inside of the resistance. Of course, there aren’t many ponies left, and those alive are left with anxiety of bein spied by inanimate objects, or worse, their own family.
 
Nightmare Moon’s timeline: All chances of victory and hope are gone, now Luna reigns over Equestria with an Iron hoof, those who defy her, or even touch the subject of her sister will disappear overnight. Everypony has already accepted living in this timeline.
 
Tirek’s timeline: I personally think that Tirek, since he already has all the magic of Equestria, has no use for ponies anymore, and he is exterminating them, leaving the land bleak lifeless, and welcoming other creatures like him with him as its undisputed ruler.
 
Discord’s timeline: Equestria has been transformed into the worst place to live in, now daily life is hell for everypony; at least in Tirek’s timeline they would die quick, but now they’ll live in a never-ending nightmare.
 
Flim and Flam timeline: Now Equestria is on the brink of collapse due to exploiting the land, abuse of natural resurces and greed altogether. In this timeline there were no other villains other than the ponies themselves with Flim and Flam adding wood to the fire, Equestria didn’t need a magic villain, Equestria gave up itself (and should make US reflect on our behavior as a race and the damage we are doing to the planet).
 
Last timeline: I have two theories…
 
1- There was a some HUGE conflict between every single race (dragons, ponies, griffons, etc…) and as a result, there is nothing left; no people, no ruins, no ideas of anything that has ever existed did… There’s just nothing left…
 
2- This is GROGAR’S timeline, and he proceeded to re-decorate Equestria as he liked it; apocalyptic and without any kind of threat or resistance to him, only he and his monsters exist now.

Nice work

@Veylon
 
it’s a crying shame those apple trees aren’t in this lineup.
 
their expressionless trunks, their judging foliage…
 
always judging me…

@redweasel  
Yes, that’s the one.

@Veylon
 
you mean the future where the apple trees turned against everypony, and only two brilliant pony engineers stood between the world and its doom?

But what about Flim and Flam?

_“Oh no, not again.”  
—Karn

@SavageSpelling  
It’s the only one where they have a chance of fighting back.

King Sombra timeline is the best timeline just because it’s the only one were ponies are not killed while their world is not complete destroyed thus proving he might not be the strongest but he is the more capable of them all.