Shibuya Sky is the one that is both open air and has a glass floor. Unfortunately, there is zero chance you will land tickets for anytime after 5pm.

If you're into observation decks, as my family is, then why not do 2 - one during day time and one at night, if your itinerary allows?

We thought Shibuya Sky after dark was absolutely fantastic on our last trip in December (didn't have any trouble getting evening tickets off their website, 1 mth in advance). We also did the Metropolitan Building during day time - slightly boring surrounding area in terms of walking to/from the building, but the view was great and we had a crystal clear view of Mt Fuji - an obvious benefit of travelling in December.

We've also done Tokyo Sky Tree twice, once in day and once at night - you can't go wrong with this option either, provided you're in the general area and it works with your itinerary.

Next visit we'll try to combine Azabudai with our visit to TeamLabs Borderless.

I recommend doing 2 also because Shibuya Sky is a different experience from an indoor observatory, and it's fantastic.

I don't recommend the Metropolitan Government Building. It's inferior to other good ones such as Tokyo City View, Sky Tree, Tokyo Tower, and Azabudai Hills.

Tokyo Skytree is a replica of the Eiffel Tower. Only taller!