When designing something for use, it is possible to measure the object's functionality or usefulness a number of different ways.  In a few cases, the engineer or client may do this based on just one criteria, but usually this is done on the basis of several factors.  This can be illustrated using the example of buying an automobile.  Factors such as safety, cost, gas mileage, looks, etc. might all factor into the decision that is finally reached.  Additionally, this weighting of factors will usually be different for each indidvidual or group making the decisions.  Examples of this include the fact theat Consumer reports will rate cars very differently than Motr Trend or Off-Raod will as each has a different set of criteria they think are most important.  Consumer Reports tends to place maximum emphasis on safety and fuel economy while Motor Trend might feel that passenger comfort and engine performance are more important.

Such will be the case in this project.  There are a number of ways one might rate the "best" paper airplane.  The simplest way to do this would be to pick a single criterion, such as flight distance, to use.  However, simple rating systems often ignore that the object may have multiple uses.  An example might be that flight time might be determined to be important for some reason.  If so, then the single factor of flight distance may discourage the creation of planes that stay in the air a long time if those planes do so by meandering around.  In this case, the judgement criteria actually works against the design goal.  Hence, it is usually very important to have the design goals in mind before setting the measurement criteria.  By doing this, criteria can be chosen and weighted to best match the design goals.

A second issure here is that of measurement.  For a person to make a claim that one design is better than another, all measurements will need to be made with a high level of precision.  One might say a plane flies 100 feet but if that number is not reproducable then it is meaningless.  A criterion must be measured with a mind to including an estimation of the uncertainty in the measurement.  In your decisions on how each criteria will be measured, this factor must be taken into account.

Some possible criteria (not an exhaustive list):

Flight Distance
Flight Time
Straight Line Accuracy
Overall Weight
Reproducability
Durability