I've stayed at all 3 hotels during COVID as well and if had to personally choose just one, I would choose the Park Hyatt Seoul. I like their hardware the best and they have the best service of the 3 and for Globalists, they have happy hour. During Summer, the Grand Hyatt Seoul has the outdoor pool which is nice but otherwise, I like the Andaz pool the best of the 3 because of the hot tubs and the sauna facilities. I think the Grand Hyatt has the best view facing the river from Mt. Namsan but the Park Hyatt view is quite interesting being at the intersection. After the Park Hyatt, my second choice is probably the Andaz with the Grand Hyatt being 3rd although I might choose the Grand Hyatt during the Summer since the outdoor pool is open.