What is the best gift a father could get and not offend anyone?

First of all you must eliminate the name father because that is sexist.

Parent does not work, because what if the child was adopted, fertilized in a Petri dish, or kidnapped?

Wikipedia defines parent a caretaker of the offspring in their own species.  They go on to add more adjectives but did not come up with a single generic definition.

 That brings up the point of the child called an offspring, Offspring implies genetics. What about the Tarzan who was raised by apes or the boy who was raised by wolves? Were they not the primary caretakers in each case? What about an adult who lives on its own?  Are its parents no longer parents?

The best gift a guardian can get is for the recipient of the guardian’s efforts to become a model citizen of the species it resides in.