Ignoring cases like Whis, where it is hard to gauge how strong Beerus was before being trained as God of Destruction, this one is pretty straightforward.
Goku is the best trainer. With one year devoted to Gohan he managed to take the kid from being firmly below Freeza to being equal to Perfect Cell(or thereabouts). In addition to helping his son gain a massive boost in power he also made sure to tell Gohan what not to do, like becoming an USSJ. Piccolo gave Gohan the basics and the foundation but Goku turned him into the perfect successor, ideally anyway. Gohan was still a pacifist but that isn't something you can do anything about.
Again Whis might be better but with Goku being a natural 6 to Beerus's 10 in BoG and still being below him in RoF means that Whis hasn't gotten him up there that quickly.
Alternatively it is whoever trained Freeza in his 4 months of rigorous training. Because oh my god.
It's much easier to train someone who is already well-adept in the use of ki and whatnot. That said, if we're looking solely at the sheer development of raw ki, then I actually think Gohan is one of the best Masters in series.
When he first started training Videl, for example, she didn't even know what ki was--let alone how to access it on any level. Yet by the end of the 1st day, Gohan had taught her to harness/control her ki enough to hover above the ground. In 10 days she was able to fly freely.
And in the film Yo! SGahFR, Goten/Trunks started having trouble with Abo/Kado when they split up because the boys couldn't read ki. But when Gohan intervened and gave them a few lines of expert advise, they immediately gained the ability to read ki, and proceeded to stomp Abo/Kado.
I don't know if it's because of his calm/peaceful demeanor or whatever, but Gohan is evidently an excellent Master.
I always thought Piccolo was the best teacher. Look what he did with Gohan.
I'll use part of Galan's ex.
Though Piccolo wasn't a nice teacher, through his training he made Gohan an animal in a few yrs. Gohan prior to Piccolo's training never used his Ki unless he was angry, and even then, he never remembered it.
He thought Gohan how to fight, not to be afraid as much. Didn't always work though, how to control his Ki for not only fighting, but also as a projectile.
He went even further in the Buu Saga with Trunks and Gotenks.
I will say I see Piccolo as the best one, and using Galan's description for putting Gohan after him.
Piccolo's training was good in the sense that it turned Gohan into less of a timid b*tch. Other then that, he just taught Gohan the basics: fighting technique, sensing ki, and manifesting energy blasts... Goku's training in the RoSaT developed Gohan a lot more, imo.
Though it's worth noting that Piccolo himself only trained Gohan for the last 6 months before Vegeta/Nappa arrived. The fist 6 months was just wilderness training: Gohan surviving alone in the wild.
Hm... I'd really say that the best trainer is probably Whiss. That's just because he apparently has a fighting style, that can make you a good enough fighter to beat people, regardless of how strong they are. Even Bills hasn't mastered it, after millions of years of training.
That's pretty impressive, imo. I like seeing everyone's thoughts here, though.