Altansar, because that's where the biggest (still living) badass in the galaxy - Maugan Ra - comes from.

Well, I was disappointed that my favorite craftworld (Alaitoc) wasn't included, so I voted Altansar because it also begins with the letter "A".

Mymeara. That's where the badass Shadow Spectres are from and Irillyth looks badass. Also I like blue.

Yme-Loc

I like their focus on heavy weapons and Titans

Reposting from announcement thread:

I've been looking at the colors, emblems, and stuff of the confirmed factions.
We already have white, black, yellow and red represented by Biel-tan, Ulthwe, Iyanden and Saim-hann.
Orange or Pink (Yme-loc, or Altansar) would add to the "hot" colors.
Blue (Mymeara) would add a more variety in that regard.

Mymeara's symbol looks like Saim-hann's S with Ulthwe Eye on top. Too little variety.
Yme-loc's Forge is kind of similar to Biel-tan's Chalice, though the focus on the flame makes it passably different.
Altansar's Chain is the most different compared to the confirmed sub-factions.

Based from twitch descriptions:
Altansar "specialty" is firepower and survivability.
Mymeara's is mobility and stealth.
Yme-loc sounds like it would focus on gear and heavier vehicles.

I think I'll vote Aringhe.

So I have no idea which one to vote for. I really have no interest in these Xeno scum so I've already put up a post into the Xeno's board and I hope to hear back from some of those Xeno's on which faction would be worth my vote. I do love how we get to choose the 5th faction. I'm hoping they do more of these for the other races as well.

All four factions get a fifth subfaction, chosen by us.

I voted Mymera, why? 'cause blue.
I have no idea who this guys are.. truly...
I just wait for chaos voting so I can get Thousand Sons *wishful thinking*

In the absence of any information about what they actually do, Yme Loc has the most interesting colour scheme.

I don't think we need another red eldar sub faction, we already have Saim-Hann.