Here's a thought I had that might make for a fun discussion. If we matched up servants' places of origins with modern countries, which countries would have the most impressive lineups? For example, the country of Georgia would claim Medea since she's from Colchis, which is now a part of Georgia. We can discuss beyond canon servants, too, and include figures like Genghis Khan who are obvious servant material and who we can infer how strong they would be as servants.
Some qualifiers to the discussion. We're considering servants' strength on neutral turf, so no geographical buffs. We're also not considering multiples of a servant. The UK can't fill their roster with different versions of Saber. There's probably some debate to be had over what countries could claim a certain servant. For example, who gets to claim Constantine the Great? I'd argue for Turkey since Constantine is most famously associated with Constantinople, which is located in Turkey today.
I feel like this kind of ends up like comparing sports rosters. You have a few countries that are absolute powerhouses like Greece that have both a lot of top tier servants along with a lot of depth of good but lesser tier servants from throughout history. Other countries might have a couple top tier servants along with a lot of depth. Tunisia is an example where they've got just a couple amazing ones like Hannibal and not much else. Other countries might have a depth of servants but none really stand out as top tier. There's probably a lot of western/American bias in my views that biases who I think are the most famous or legendary figures.
I'm pretty convinced that Italy, Greece, India, Egypt, Iran, Iraq, China, and Israel/Palestine are surefire powerhouses, the Real Madrids if you will. They have a ton of top tier servants from ancient history down to the medieval period. Along with that, they all have a lot of depth servants from all historical periods.