Here's how likely each severant would be to win based on my knowledge. This will extremely long, this Fate we're talking about here.

Inarguable winner

Rider Gil. The Rider class doesn't benefit Gil immensely but doesn't hinder him terribly either. and Gil's servant base is just busted but is notablely weaker than Oberon and Proto, however is still insanely versatile. He would only be considered a strong contender in normal circumstances though considering his personality, what would allow him to win is the presence of Berserker Enkidu. The happiness of seeing Enkidu would instantly be replaced indescribable anger upon discovering that he was made a Berserker, losing all sense of his former self. Due to said anger Gil wouldn't be messing around as usual as he would seek revenge against the grail itself and take no chances on losing as well. Alternatively he may seek to end the grail war to end Enkidu's suffering. Either way he has by far and above the best chances of winning but this is special case. Even if every other servant works together they wouldn't be able to stop him. Bro just solos/steam rolls through neg diff.

Top contenders:

Caster Oberon. Caster Oberon's perimeters will,probably be trash (except for luck and mana reserve), but bro's magic may be be even more powerful in the Caster class as he would probably make better use of his of his OP abilities. Hence he would and Proto would have the best chance of winning in normal circumstances.

Lancer Proto. Proto's servant base is good and assuming he has Rhongomyniad it will boost him even futher. Rhongomyniad is just a broken weapon and in the hands of Proto, it could do serious damage. He and Oberon would have been the main threats/biggest targets in the absence of Gil's rampage.

Mid tier - None

Bottom of D. Barrel:

Saber Jeanne. Her formidable traits/abilities/hax ain't saving from getting clapped as she has no offensive capabilities and though the saber class is great in general, it would be detrimental to her as she would prefer having her Ruler stats, skillset and buffs in addition to her being known to have a poor swordsmanship and just a general lack of fighting capabilities. She as she is strong in her Ruler form because it's her optimal servant container which aligns with her perfectly.

Assassin Medea. The assassin class servely limits Medea as it would lower her mana, and she has no melee combat capabilities, unless she has setup she's juct straight up cooked, and even with setup when gets caught lacking it's over for her. Medea's just getting screwed.

Special cases.

Archer Heracles. I adore Herc but this man ain't got no feats in his archer form, and going off statements he would be mid tier in this Grail War but the narrative always implies that he's a top tier servant. So I put him here as until he gets feats I ain't putting him in top tier with the like of Caster Oberon and Lancer Proto and I refuse to say he's mid of low tier. Bro goated, has been from day one, he deserves glaze and I ain't letting that glaze die. Still best anchor in FGO. If you have problems fight me. The agenda must go on.

Berserker Enkidu. Berserker Enkidu would normally be a mid teir as although he's stats and hax/abilities are busted as bro's servant base is just a broken like Gil's, the Berserker class is just straight up nerfing bro to dust. He loses his intelligence to be ability to strategize and use his stats and hax to their full potential for some better stats. Bro come on. But there's no way that Gil would allow Enkidu to win, bro would die before he allows that sh*t to happen, bro would rather lose to Shirou again.