Chrom and Sumia just because it's easy to max out and they have a decent support. Being able to get A rank really early can be nice with the extra boosts they give each other.
Lucina and Gerome because they have a neat history with each other and I like the contrast between someone with a wyvern murdering sword and a dude who rides a wyvern himself.
M!Robin... depends on boon and bane but usually Cordelia because of her good genes, access to armsthrift (I'm stingy) and just being a strong unit. Can't go wrong with her.
