IF she ever marries/stay married (it's possible she ends up as Tyrion's or whoever else's widow and never remarries, as a sort of Elizabeth the First) and IF he survives (I'm not sure about that), then I think Tyrion will be the best choice. Politically, it would do only good to Westeros... it would unite Starks and Lannisters, as it happened in real history when the War of the Roses ended with Henry the VII Tudor (descendant of an illegittimate branch of house Lancaster) marrided Elizabeth