EliKiyo is one of my favorite FGO ships - there's such an interesting dynamic to them, especially with Riyo's portrayal of them being rivals-turned-friends. It's a shame that there's barely any fanart of them interacting, and even less so of actual shipping.
I don't know why, but I've always liked the Reimu x Yuuka ship, even though it's so rare and obscure. Actually, come to think of it one of the first Touhou comics I read was a Reimu x Yuuka one, so maybe that's why.
Flandre being grumpy and embarrassed with Koishi being carefree and happy is a nice dynamic. Especially since it can
lead into shenanigans with their elder sisters spying on them like in this picture.