First you need to deside what the purpose of the test is. Is it to get an estimate of the exact fitness level or is it rather to track changes over time.
Secondly, you need to define how much time an effort that can be put in to the actual testing. Finally, the available skills and equipment will set the limits for whats possible.
To measure your fitness level
If maximal effort is acceptable to you, the watt max test is an excellent test that will yield reliable results, I fyou are not accustomed to bicycling, you could instead choose the Bruce treadmill test. If you on the other hand need a submaximal test that isn't to streneous, the new steptest will be a good choice.
To track changes over time
Again, the two maximal tests - The watt max test and the Bruce treadmill test can be used reliably also for this purpose, but if you need a test with lower intensity, we suggest you take a look at the classic Aastrand test. The Aastrand test will, when conducted carefully, be very good for tracking changes in capacity over time. If a bicycle ergometer is not available, you could choose the One mile walk test.
Other tests are available and each has its strength and weaknesses. Whatever you choose, one factor is crucial: you should be very careful to perform the test under exactly the same conditions every time.