Saint martha(ruler)

Pros:
- Ruler class: takes 0.5 damage from any class besides berserkers and deals 1.1 damage against everyone besides berserkers (2x)
- Buster deck: expect to deal a decent amount of damage
- Heal + Ruler = great staying power
- Absurd attack buff against Demon, Divine and Undead enemies
- NP reduces defense before damage
- NP charge skill

Cons:
- Ruler class means she deals mostly neutral damage
- Attack buffs are inconsistent (1st skill dependant on water field and 3rd skill lasts for 1 turn)
- Triple buster deck means she has average or below average NP gain

Love Perspective:
- Martha has nice selection of ascension outfits.
- Martha fists her enemies to death.
- Martha is JoJo incarnate with her NP.

Gameplay Perspective:
- Being a Ruler with the cost of 4 star means she can easily fit into any team.
- She can tank a lot of damage because again she's a ruler, but also has 2 heal skills and has an NP gain skill as well.
- She's an offensive ruler with a 3 buster card deck so she hits better than Jeanne and Shirou.

As for a tierlist of Swimsuit units, you can view it in the 4 star tierlist on this site. Personally I think: Squirtoria>Martha=Tamamo>Moedred>Anne and Bonny>Kiyo>Marie

She's nowhere near the worst. Imo the breakdown, gameplay-wise, is something like this (best to worst):

- Arturia: One of the best ST archers in the game (if not THE best); extremely spammable NP that may also reduce enemy's NP gauge, with good skills and almost no real drawbacks, other than the fact that her second skill (heal) also saps some of her NP. Not much of a problem.

- Tamamo lancer: A very good ST lancer, which for some reason are pretty rare to come by. Excellent damage against Male archers and berserkers (most of them are male) with her NP. Two of her skills come with annoying demerits, but they aren't that bad, and considering how good both those skills are the demerits aren't that much of a problem. She also has pretty decent survivability.

- Martha/Kiyohime: Both great for different reasons. Martha is just about the only damage-oriented ruler, as well as the only 4* ruler. People have already gone over her ruler class advantages so I won't bother to do that. Her main drawback is just that her damage is kinda situational: great against divine/undead/etc, not so much for most other things. Kiyohime is a straight damage lancer. Madness enhancement EX gives her higher damage than most, and her skills are focused on helping her do as much damage as possible. Drawback to her is that she lacks survivability.

- Mordred: Good AOE farmer with a spammable NP. That's about it.

- Anne & Mary: ST archer with a very gimmicky NP. Does A LOT of burst damage when used right, but will likely die soon after unless you've got some kind of countermeasure for that.

-Caster Marie: Worst summer servant imo. She isn't horrible, just not as good as the rest. Her survivability is right up there with Marta's since she has a 3-hit invincibility on a 6-turn cooldown, a slightly worse version of Cu's protection from arrows. Other than that she's mostly just support with a low-damage AOE NP.

If you are rolling, I recommend rolling when either Tamamo/Kiyo or Arturia/Martha are on rate-up.