I would have to say the ground, mountains, and water. Trees too.
They're most outdated in FE:LH i believe.
Then nametags of cities is outdated as well, It simply don't have enough text space for long names.
However... the catch is if you overhaul the graphics of world then it's the units and cities that look outdated next
In my case, this is very difficult of a question to answer. Because I'm very tolerant of graphics of the game as long as the game is very fun to play.
Because you see I'm still playing skies of arcadia from it's release date in 2000 and it's 2014 now.. but even then I play skies of arcadia less and less due to uh obvious lack of new content And graphics for skies of arcadia don't feel outdated to me cuz it's very fun to play.
Ok I figured it out, if the game becomes boring to play then it's graphics will start to look outdated to people that got bored of such games.