Re: What are the most accurate weather instruments? Why?

The most accurate weather instruments are those that have a long durability
and increased precision. For example, an instrument that records
temperature to within +-1 degree and humidity to within +-1% is not
as accurate as one that would do so at the +-0.1 degree or +-0.1%.
They also have to be repeatable, 0.2 inches of rainfall should always
read as 0.2 inches of rainfall under a variety of conditions.
Advertisements from many different companies will show you that who
has the most accurate instruments is a debate that is not easily won.
Davis Instruments
Climatronics

But, some of the most accurate, durable instruments are satellites
now used for most weather forecasting.
They are capable of making measurements to VERY high accuracy.  And 
their capability to monitor the earth and atmosphere in many different 
wavelengths of light allow them to produce many products that give very
detailed information about the atmospheric conditions that temperature,
humidity and wind speed leave lacking.
GOES Satellites

Homemade instruments can be very reliable if calibrated well.
In order to evaluate the accuracy of your instruments you should test the
values you get from them versus your own local weather forecast.
You could create plots of differences (yours-forecast) versus time
for each instrument over the course of a few days (or weeks) and judge 
for yourself.  If your local weather is not that local, a bank time and
temperature call in line or weather report from a small local
airport would suffice.