Saber Diarmuid, he is the one that you want summon unlike his lancer counterpart since he is stronger than his lance variant
He defeated post shimousa musachi, which is something not many mservants can do with just pure blade skills alone
Uh, that's not what happened... someone really didn't read through Enma-Tei. The issue wasn't that only Diarmuid was able to defeat her Shadow but the fact that she refused to concede against anyone not a dual swordsman like she is(aka having a pissy fit).
And again, it's hilariously overstated how strong and skilled Musashi's swordsmanship is especially considering that characters like Karna go well beyond that as well as Siegfried and Sigurd having skills that goes "beyond humanity" like Musashi does.
Diarmuid low-medium difficulty. He was matching Saber in pure skill quite handedly
In the Anime? Sort of. In the LN? Absolutely not. Diarmuid was actually having issues blocking her attacks without his own body feeling numb from it and that's with Urobuchi's Artoria whose basically weak as shit and loses fights to Horrors without help. He even only got as far as he did because Artoria made a stupid judgement call she should have never made from the start.
and has extremely high agility for a Servant, even higher than Cu
Agility as a stat is overrated as fuck and I have never seen or heard of Diarmuid being a "fast boy" especially in comparison to Cu who is consistently called an extremely fast Servant, the fastest Servant in the 5th HGW, etc.
EDIT: To make it more obvious, Agility as a stat is more the combination of reflexes, bursts of speed, endurance, dexterity, etc. than JUST how fast you can move.
Hessian Lobo has A+ Agility but it seems primarily put into his Reflexes and how long he can go than being a truly fast wolf.
Cu beat Emiya in UBW if I recall correctly and Diarmuid is stated to take the edge against Cu unless they were fighting in Ireland.
1. Cu defeating EMIYA =/= Diarmuid defeating EMIYA, that's not how debating works.
2. That was literally only stated if Cu has Kotomine as a Master and Diarmuid has Kayneth as a Master and Kotomine explicitly doesn't have the Magical Energy to keep Cu around like that.
Diarmuid's magic nullifying spear ought to be able to pierce Rho Aias like it bypassed Saber's armor.
Why the hell would EMIYA use Rho Aias against Diarmuid's NP? Does he forget that Diarmuid cannot chuck his Spears as an NP unlike Cu and Scathath? Neither are Gae Bolgs...
Secondly, this ignores that while yes, Diarmuid's NP can potentially screw with EMIYA's Projection, it can only do so on contact and even then, it loses that the second it's removed and even then, strong enough Magecraft isn't instantly dispelled either.
Like I can understand going with Saber Diarmuid here but Lancer Diarmuid is not in EMIYA's ballpark.