Who is the best trainer in DBZ?
Goku, Piccolo, or Future Gohan?
Let's look at who they've trained and what those people have learnt from it.
Goku trained Gohan. He brought the Super Saiyan out of him and taught him not to be afraid of switching and releasing his full potential. He also taught him how to remain in Super Saiyan with minimum energy consumption. He brought Gohan from being weaker than Frieza to stronger than Cell in a year.
Piccolo trained Gohan, Goten and Trunks.
He taught Gohan all the basics and got him to a level where he was a competent fighter (AT THE AGE OF 5!). Taught him techniques such as Masenko and other energy based things such as Energy Shield. He showed Gohan that life is tough - which is an important lesson. He taught Goten and Trunks the Fusion Dance (with help from Goku) which put them from being weaker than Frieza individually to being on par with Super Buu. That being said, Goku had the capacity to do that too (though it would have taken him longer).
Future Gohan trained Trunks.
He taught Trunks to let his feelings fuel his power. Because of this, Trunks burst into Super Saiyan. If we take the manga to be canon then he did it before Gohan dies (which is a testament to Gohan's teaching abilities) but if we take the anime to be canon, then he did it after Gohan dies. Future Trunks does not learn anything else from Gohan (though he didn't have much time).
In my opinion, it would be Goku. This is because Goku is a genius and has ways to bring out the best from you, which is a contrast to Piccolo's cold (but effective) "grow up" approach.