From Webkinz Insider Wiki
Buying foods wisely can achieve great results - one example is using the boosts below to determine what foods to feed pets at certain times. For example, if your pet has a low happiness boost but full hunger and health boosts, just feed it some marshmallows which will bring up your pet's happiness continuously without ever filling it up!
WebkinzInsider's thread for boost value information (Link) is the source of all of the data provided here. All of the information has been tested and verified by WebkinzInsider members.
Click the links below for all boost information.
On July 8, 2015, a major update titled Webkinz X was introduced to Webkinz World. It included an overhaul of the meters and boost values, rendering all pre-Webkinz X data useless.
The WIki still retains the old values for historical interest.