Best SSR Saber?

Depends on what you want them for, they're each the "best" at different things. If you don't like Musashi, don't roll for her - I'll leave her out of this comparison.

To start off with, there are, for the foreseeable future, 2 other ST SSR sabers: Nero Bride and Okita.

- Okita is a quick oriented servant who is incredibly self-sufficient. Pros are she can generate tons of crit stars pretty easily, and can reliably absorb them to crit a lot. This means she also fills her NP gauge more easily than many other servants. She has a dodge tied to her crit buff, a quick buff for one turn, and a star absorption skill, all of which are incredibly useful for her to have and contribute to your goal with her:
1. Generate lots of stars
2. Absorb them and crit hard for massive damage
3. Fill NP Gauge
4. NP
5. Repeat
6. Profit.

Not many cons with her honestly, other than maybe the fact that her crit buff and dodge are tied to the same skill, so you'll need to sacrfice the timing for one to use the other.

- Bride is a support-DPS hybrid; all three of her skills are incredibly good, incredibly useful targetable buffs. She can support other servants with them or use them on herself to become the main DPS, making her one of the most versatile servants in the game. A common tactic with her is to put her in your 4th slot. That way, whichever one of your main three servants dies, she can come in and essentially fill in for their role.

Con for Bride is that she lacks any hard defence like dodge or invincibility, but that's not really much of a problem.

As for AOE sabers, they all play relatively similarly and for now are mostly only good for farming. I'll make it brief:
- OG Altria is currently the worst since she doesn't really stand out among her competition, but later on she gets strengthened giving her an NP charge skill that makes her much, much better, possibly the best of the three.
- Mordred has a useless instinct skill just like Altria but she does have her own NP charge skill already, and the ability to remove debuffs and buff defence, which are all pretty useful.
- Altera has the best damage of the three right now and at least for the purposes of farming, she's the best, although her lack of NP charge means that she falls off in this role when Altria gets strengthened.

For now, for STs it's really not a good idea to make a suggestion for you without knowing what you want, I'll leave that up to you.

For AOE, for now:
Mordred >= Altera > Altria
After strengthening:
Altria > Mordred >= Altera

Another user already posted an analysis on the current SSR Sabers, so all i will do is point out that there will be 2 more coming in the future.

Proto-Arthur is for the most part just an OG Seiba who lost Charisma in favor of Gigantic beast hunting, a way more niche skill that will be useless in the vast majority of situations, so you might as well just use the OG. Roll for him if you like his design, but he isn't really meta-defining.

Sigurd is IMO the best Buster Crit Saber in the game, if you want to run Buster Crit and don't want Musashi, he is definitely worth your SQ. He does lose out against her when it comes to survivability, but he is all around way more useful thanks to his deck being BBAAQ, so he can actually use his NP more than once in a fight, unlike Musashi. His anti-dragon niche isn't really expanded on, but he is an excellent damage dealer worth building a team around. Only problem is that he probably won't be in the NA version until 2020, but at least that leaves ample time to save for him.
Also, this isn't really important in-game, but Sigurd and Proto-Arthur also have the highest attack values of any SSR saber, and the 5th highest overall, behind Jalter, Cu Alter, Kintoki and Dantes.

Generally my suggestion would be:

ST
=>If you want to run Arts teams, Nero Bride
=>If you want to run Quick, Okita
=>If you want to run Buster, Sigurd

AoE
They are all pretty comparable, so no real winner here. Run whoever you have/want. Void Shiki shines best in arts teams though, while the others don't really require a specific team to perform well, so keep that in mind

Not Musashi? You could be great, you know, it's all here in your head, and Musashi will help you on the way to greatness, no doubt about that.

The only 2 current ST Sabers that I can think of are Umu Bride and Okita, and they're both limited. For AoE Sabers, there is Mordred, Seiba, Bad Civilization, and Shiki, and Shiki is limited. Of these, the only one I have is Okita, but she is outstanding. I would highly recommend trying to get her when Nobufest comes back around.

Don't look at the 5 stars, look at the 4 stars, look at Lancelot. He is the true single target king. Forget the 5 stars, look only at Lancelot.

But allegedly Mushashi is the top ST Saber if you can figure the best timing on her skills.