The land value should be taxed, but it isn't a simple answer. Large buildings imply more people which mean more use of city services. I'm not sure what the right answer is, but I am sure simple answers that would fit into a comment box here are wrong.

If you want to tax things to cover the cost of more people then it makes more sense to tax the people.

You obviously need to tax enough to cover the costs somehow.

Tax is also incentive. You want to tax things you want to discourage more than things that you want to encourage.