Eliminate any fans with a blade pitch less than 10 degrees. These fans will slice through the air without moving much air at all. Because the purpose of the fan is to move the air around, fans with a blade pitch less than 10 degrees will be extremely ineffective.
Eliminate any fans with a blade pitch more than 20 degrees. These fans will push a lot of air around, but they will meet a lot of resistance. Even with a very strong fan, the resistance will wear out the motor sooner than it should.
Consider the remaining fans. Make sure that the motors on the fans match up with the blade pitch. For example, a fan with an 18 degree blade pitch should not use a small (153mm) motor, or the resistance will wear it down. If the blade pitch is closer to 10 degrees, it can have a smaller motor; if it is closer to 20 degrees, it should have a larger, stronger motor. Most quality fans have a blade pitch of around 14 degrees.