I've often felt that she stays with him because that's what her morals tell her to do (RE: marriage, the kids). That's not to say that she doesn't still love him, but it's not what it once was. With his head always in his work, Reed's never there for her. That can't feel good. And remember, Sue fell for Reed when she was 12 years old. He was boarding at her aunt's house -- and was at least twice her age. It was a schoolgirl crush.
Despite being a king, Namor would live to serve Sue. However, he might not be the best catch either, seeing as how he's something out of a fairytale -- being the handsome prince (in this case king) -- and might not be able to live up to his idealized potential. But they both clearly long for one another. Even if it didn't last forever (RE: "...happily ever after"), it would be true love.
So my vote goes to Namor.