I've only played the GC and Wii Fire Emblem games, so I can't wait to get this. The best unit is most likely going to be the main character. :P Ike in the Wii FE game can take down the entire game without ever getting hit, and before his first turn.
It's not so much that certain types are always better to have than others, you want to diversify and split your team into smaller groups who should normally travel together. Hide the weaker mages and archers behind your "tanks" and you'll learn how to set up good positioning on the map depending on the situation.
The main question isn't what type of unit you should have, it's which individual person you should have, as that tends to make a way bigger difference. Unit type barely seems to matter in the end as long as you have a diverse lineup (which you will have pretty much no matter what). Just make sure you pick the best characters!
