Grand Hyatt is one of my favorites in Seoul - slightly older than some of the newer 5 stars so rooms slightly smaller. But near Itaewon, if that is you're thing. Not sure why tripadvisor ranks so low, maybe cuz it's pricy.
Westin Chosun is one of my favorite places for business breakfast meetings - used it often when staying at the Lotte Hotel next door. Never stayed in their rooms. Located in old central area, next to myeong-dong.