Grand Hyatt is excellent but older hotel (smaller rooms), favorite of IB guys. Good pool, good food.

I'm not sure which of the Hiltons is nearby - but that's the ones that has problems getting taxis. Used to be a very hot disco there - not sure these days.

Both are far from subway and north of the river in the older part of the city.

If location is important, stick to Kangnam area - Coex complex - there is the Grand Intercon, the Coex Inter and Oakwood Premier.