Hotel lobbies have personalities. Like people, you may find some of them to be just a bit too energetic and loud and seemingly wanting to keep the conversation going in all directions at once. Others do look nice and impressive but digging just a tiny bit underneath the surface, you end up finding nothing of interest. And others yet may seem to be nothing at all, but then you stay anyway and eventually find yourself immersed in a friendship you never saw coming. Sort of. I really don't know why, but I find lobbies interesting on so many levels.
My favorite in Tokyo is the Ritz-Carlton one. Especially when they don't do live music. It's like a giant living room, time runs slowly, conversations are easy. The couches are great, especially the one over in the corner facing the windows. Watch the Tokyo skyline from the 42nd floor late at night, share a pastry from today's cake selection. Go nuts.
If you opt for the subway mode of transportation, head for the Roppongi station on the Hibuya (H04) or Odeo (E23) subway lines. Follow the signs towards Tokyo Midtown and enter the hotel through the small plaza entrance right next to the Ritz-Carlton pastry shop, or walk around the complex to the hotel main entrance facing the Midtown Garden.