All three are fine, but location may be a more important factor. The Radisson (people in Seoul call it the Plaza) is in central Seoul (close to cultural attractions), while the other two are in Kangnam, a new area south of the river, home to tech and financial companies, apartment blocks, and upscale nightlife. Commuting between those two areas by anything but subway is unpractical.

The Ritz is probably the poshest of the three. At the Plaza, there is quite a difference between the nicer rooms and the smaller ones targeted at Japanese weekend tourists; the more attractive prices you will find usually mean the latter.

While opinions differ, I would recommend to a couple on a sightseeing visit to stay downtown, rather than in Kangnam.

This thread may provide further insights:

http://www.flyertalk.com/forum/Forum74/HTML/005942.html