Kaze in Birthright
Xander in conquest
My first S support ever was Azura and Jakob in Birthright though
To be fair I always married Robin and Sumia as well, and they have one of the best romantic supports in Awakening, not sure why this pairing isn't more popular, maybe because a lot of people find Sumia bland so she is shadowed by girls like Lucina, Tharja and Cordelia?
As for Leo , particularly never cared for him but isn't him the most popular Fates character in Japan?