I can see why other people might not like Chrom/Sumia, but I keep getting it because, well... Pega-Pony-Princess. Come on.
Let's see... I kinda dislike any pairing for Nowi that isn't Donnel, Ricken or a young-looking M!Avatar, because even if she's chronologically thousands of years old, she looks and acts like a little kid. In my first playthrough I didn't plan things ahead and paired her with Frederick, and it was just really awkward to watch. I like some of Nowi's supports, particularly with Libra, but only up to A because the friendship is cute. It doesn't help that she acts so childish in the supports. If she gradually showed more maturity it might be more okay.
I also got Chrom/F!Avatar on my first playthrough by accident. I don't think it added that much to the storyline, except more angst for poor Lucina, who didn't need it.
Most of Cordelia's supports annoy me with her obsession with Chrom, even though I mostly like Cordelia. I guess Libra wasn't too bad with her though. Getting her to stop overworking herself was sorta sweet.