Honestly, Chrom x Olivia still remains as a best pairing for Chrom. The only rival would be Cordelia, but most of what she has to offer Lucina(Vengeance, Armthrift, Sol) aren't that good compared what Olivia gives(Vantage, Swordfaire, Lethality for Streetpass). I know most people overhype Vantage and think it's useless on Lucina, but it's generally better than Cordelia's skills(And honestly, Armthrift and Sol are way more overhyped). And Severa gets cheated out of Vantage to compliment her initial Vengeance.
Sumia doesn't honestly care, since her child's basically perfect. The best would probably be Gregor, since none of the other children really need him(Lon'qu can replace him for Laurent and Maribelle also doesn't care who she marries).