Lon'qu x Miriel = Win js. Miriel can run around as a Dark Knight and hit everything with magic and Lon'qu finishes the job nicely with a Killing Edge and crits very regularly, and you can always have Lon'qu attack if you're facing something with high Res. The combo isn't that amazing in the hardest DLC chapters, but it's still very good for main story stuff.
In the file where I had a male main character, I married Anna, because hell yes Anna is amazingly cool. My main file has a female main character so I married Donnel and got a ridiculously OP Morgan thanks to Aptitude.
The best characters you can get in the game are the children of your marriages who appear later in the game (after Chapter 13 iirc). The best ones, fyi, are Lucina, Cynthia (Sumia's daughter), Morgan (Main Character's daughter and arguably the best character in the game), and Owain (Lissa's Son).
Also, everyone praises the hell out of Donnel because Aptitude allows him to get high stats very quickly. While that is cool, in the long run he isn't much better than anybody else in the game, and seeing as this game is nowhere near as tedious to grind in as most RPGs, it's not hard for me to grind my other characters, and they all max out with high stat totals. Donnel doesn't have better stats, he simply gets better stats faster, and while that is good if you don't like level grinding, I personally think he's overrated if you're like me and just want a ridiculously OP Streetpass team.
Oh right, and I'm halfway through DTB Season 2. I still stand by my original opinion.