Musashi

Considering Buster Memes with Merlin will be meta for a very long time? Yes.

Really though, she is very good. She is a Gorilla Buster servant who has very high single turn burst damage, as her first skill drastically boosts her star gen and NP gen, because it doubles her hitcounts (note: this skill halves her damage per hit at lvl 1, but at level 10 each hit deals 80% instead).

Her second skill is essentially your average Mana Burst (ups Buster performance 30-50% by level) but also has Invulnerability Pierce on it, which also bust through evades, so she will always connect on the turn you use it.

Her third skill gives her Invulnerability for 1 turn, removes debuffs, and also increases her stargen by 10-30%, so it's pretty good as well
(Note that all her skills are 1 turn only, the only exception is the stargen boost, which is 3 turns)

Her NP is a simple ST Buster with 7 hits, and its overcharge effect is bonus NP damage.

Overall, she is really good as a Buster servant and has very good and somewhat consistent damage output, but her main deal is one turn Buster Burst damage, preferably boosted by Merlin. Also, without her first skill up, she has a bit of trouble generating NP and Stars due to only having one Quick and Arts card, so she does need a bit of support to help her reach her full potential

Highly recommend if you have Merlin then have her. Indeed she is the strongest ST saber but it is stronk Musashi + Merlin + Merlin Comp > Any others. When a perfect NPBB critical brave chain with all Buffs on (self+dual Merlin), Musashi can provide >800,000 damaging to opposites classes. Even a normal round Mushashi's Buster card can provide decent damage as Berserkers.
Teamed up with Waver, Nightinggale or Hans can also be cool, but the effect could be somehow fade than with Merlin for Musashi, especially Merlin with 4060.