Usually your originally stocked fish are going to have the best growth rates and provide the best opportunity to grow a trophy, but this isn't always true as Eric stated. However, the first stocked fish have the least amount of competition and therefore far more resources than subsequent generations that are actually born in your pond.

If a pond has had year and years of "cross breeding" between a limited number of original parent fish, this can lead to inbreeding and reduce the growth rates and size potential of fish. When stocking a pond, choosing to stock species from different sources may be a good idea for those fish you plan on naturally reproducing in your pond. Often the removal of a certain percentage of a certain species of fish from a pond and replacing it with new genetics from an outside source can help solve some of the genetic inbreeding and increase growth rates and ultimately size potential. Usually the biggest factor in older well established ponds is the simple lack of food. Too many fish and not enough resources to feed them all... This is often seen in unmanaged ponds with LMB.