6th Singularity Camelot: Land of Emptiness free quest is the best AP & time-saving location to farm Meteor Horseshoe. You'll get about 4~5x Meteor Horseshoe by spending 220AP.
3rd Singularity Okeanos: Caldera Island free quest is the first location you can farm Meteor Horseshoe.
Here is a list of servants that need Meteoric Horseshoe to perform Ascension and Skill Up.
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
5 Star Servants Tier List: Best Servants [5/9 updated!]
FGO Reroll Tier List [5/11 updated!]
1-3 Star Servants Tier List
Best Servants [★4]: 4 Star Tier List
Best Craft Essence: Craft Essence Tier List
Cu Chulainn (Alter, Berserker)
Nero Claudius (Caster, Summer 2017)
Minamoto No Yorimitsu (Lancer, Summer 2017)
Oda Nobunaga (Berserker, Summer 2017)
Hans Christian Andersen
Frankenstein (Saber, Summer 2017)
Sometime in Summer
Piece of Year 2030 (A Fragment of 2030)