Why is steam used to spin turbines? The ideal gas law tells us that the relationship between Pressure (P),  Volume (V), Temprature (T), and quantity/amount (n) is
$$ PV = nRT $$
where R is the gas constant.
Say your an engineer who only cares about efficiency (cost is not an option). Your general goal should then be to find a fluid which, given a fixed volume, is under the most pressure for all temperatures (or the range of temperature for a given fuel). Any material then, with a greater atomic mass number with a less then or equal to boiling point then water would seem to be a better fit.
So I'm wondering, is water really the best tool for the job?  
 A: The first question is, why use a steam engine rather than an air engine?
The most efficient thermodynamic process is the Carnot Cycle - though no one has built a Carnot Engine yet. Take a look at it's T-S Diagram found there. An important feature of the Carnot-Cycle are the isothermal expansions and compressions happening.
Now, a Rankine Cycle is (in Theory) as efficient. This cycle uses steam, the evaporation and condensation of the steam are isothermal processes (in the ideal case). Rankine Engines can theoretically achieve Carnot efficiency.
Now, Water is uses for it's abundancy and high enthalpy of evaporation/condensation in many applications. However, if only lower temperature heat sources are available, other liquids with lower boiling points may be used in then so called organic rankine processes.
A: 
If your an engineer and your looking for a material to spin your turbine, your goal should be to maximize pressure and minimize temperature.

Well no.  Temperature is generally a constraint of your heat source, and your goal is to maximize profit.  That goal does, however, map to physical properties in logical ways, but it's much more complicated that what you've taken it for.  To maximize profit, that means you want to produce more useful work with less fuel input and lower capital cost.  That breaks down broadly into two categories, maximizing efficiency and lowering component cost.  A reasonable proxy for component cost is size, although, the performance requirements also play a factor, but it's complicated.
Efficiency is much more of a product of the cycle type than the fluid.  Using H2O, you commit to using a Rankine cycle (because water is liquid at room temperature).  An alternative is an all-gas Brayton cycle.  There are meaningful differences between these two routes, but what matters even more is the temperature you're able to get, which is a different question.
To a first approximation, the turbine pressure for a Rankine cycle is dictated by your boiler temperature and the saturation line of your working fluid (water).  You don't have a choice of the temperature you use.  For one, combustion reactions like coal have a defined maximum temperature.  Nuclear plants have material limits.  These can be changed, but only by adjustments to the reactor fuel and reactor type.  It's fairly separate from the selection of the thermal cycle working fluid.
For a Brayton cycle, if it's ideal, the boiler and condenser temperature determine the efficiency.
In other words, at this point we don't have any physical reason to think that the working fluid will have any impact on efficiency at all.
Moving on, non-ideal considerations will make a difference.  Basically, a turbine isn't a perfect ideal expansion process.  These factors are related to the working fluid, but probably not like you think.  Some losses come from the fact that the fluid works its way around the blades in the turbine altogether, so it doesn't make energy.  Some losses are just due to aerodynamics of the blades itself.  Some losses are even because the temperatures are so high that blades require active cooling, necessitating some mixing that harms cycle efficiency.  Some losses are because thrown away kinetic energy at the end of a turbine, to move it into a pipe to go to the next turbine.
Are all of these affected by the fluid?  Yes, but how can you say that one is better than another?  That's quite difficult.
Water is obviously abundant, so a good question might be to ask why we don't just use air (which is also abundant).  In a sense we do for direct cycle oil and natural gas cycles (can't for coal because it would muck up the turbine).  But we could also build a closed cycle based on Nitrogen gas.  One good argument for water over air is component cost.  The condenser is a very expensive component, and with water Rankine cycles, you get a lot of cost reduction because there are tons of little droplets of water flying around, aiding heat transfer.  Also, you have smaller pumps to move liquid water, as opposed to lukewarm air.

EDIT: arbitrarily through reading I stumbled on some good images of the non-ideal turbine losses.
Edge losses:

Stall eddies:

This is only one small part of what I was talking about, of course.  This is also for a compressor, so I'm kind of lying when I say the above stall condition eddies are relevant.  It still has aerodynamic losses, which are similar.
A: The question is fundamentally incorrect, given that the ideal gas law does not apply with steam.  In order to be considered an ideal gas, the fluid has to be far away from the liquid vapor dome.  In many Rankine cycles, the steam is actually partially steam, partially water.  This means it is in the liquid vapor dome.
