I tended to switch off between Marth and Roy. Roy if I wanted to do some serious damage, or if I was fighting multiple people. Marth if I wanted grace (read: awesome air attacks), or if I was dueling. Also depended on the stage (on air stages like Big Blue, Marth all the way).
Even in SSBB, I main the Fire Emblem guys. Apparently Marth was nerfed for that game, but if he was, it was in all the ways that didn't affect me; not to mention they made Shield Breaker less redundant, and they made Dolphin Slash and Dancing Blade useful. And Ike is just awesome.