What is the most complex legal field?  I would argue that it's taxation followed by Indian Law.  If you are looking for the most complex single issue, your best bet is taxation issues in Indian Country.  The AZ Supreme Court just decided the Southpoint case and the result has implications for power plants or other energy facilities built on Tribal Lands.  The case is unbelievably complex.  The tax court ruled that Mohave County could tax Calpine's Southpoint plant even though the plant was on a Reservation.  The Court of Appeals reversed the tax court and then the Supreme Court reversed the Court of Appeals.

Here's the result.  

¶37 Here, South Point indisputably owns the Plant, and the property taxes fall solely on it and not the Tribe’s land. Consequently, § 5 does not exempt the Plant from taxation, and the court of appeals erred by holding otherwise.

Here's the case.