There was a discussion on Reddit about which servant would be the best to summon for a Grail War. Tamamo Cat was mentioned. That's a good point.
There was also a distinction made between which servant could actually win a Grail War and who would be good to summon because they won't betray you and/or unleash chaos. Gilgamesh could beat most servants but he can be caught off guard if he doesn't take his opponent seriously. Plus it's difficult to win his loyalty.
Karna is in the same tier as Gilgamesh, but he's far more loyal. Karna might be easier to get along with, but he uses tons of Mana. Enough that the strain could kill his master. That almost sounds like something Urobuchi would write, lol. Wouldn't that be horribly ironic? "My servant is a demi god but he's extremely loyal and follows my order to keep casualties to a minimum. In another lifetime, we could've been good friends. I might actually walk out of this alive. Hell, I might actually win."
*Cue Karna using Brahmastra Kundala and accidentally killing his master.*
Speaking of mana drain, a couple of Berserkers seem appealing until you factor that in. Like, Kintoki, or Hijikata. (Or Tamamo Cat, like I mentioned at the beginning.) Regular Lancer Cu Chulainn during a Grail War in Ireland might be a safe option... Or assuming that the Mana drain wasn't fatal, Berserker Cu. No, not even Cu Alter. Regular Cu summoned into the Berserker class. Even though that's supposed to be a bad idea in canon... Although, Cu Alter is surprisingly loyal to his master. But that makes sense given how much normal Cu hates betrayal. So Cu Alter might be a safe choice. Again, assuming that his master has enough Mana to support a Berserker. I don't know how you'd be able to summon Cu Alter specifically, but whatever.
And if you could somehow summon her as a heroic spirit, and you get along with her, Scathach is probably a safe option, too.