Gin has always been a character to hide his actual power, so it is quite hard to judge his strength.
All we know is that:
In the rescue Rukia arc, Gin was strong enough to toy with Hitsuguya (who was raging, so maybe not as calm and collected as he should have been)
In the turn back the pendulum arc, Gin was strong enough to easily demolish the 3rd seat, when he was a 5th (so was probably hiding his power from everyone outside of Aizen)
Aizen didn't know Gin's bankai's true powers until he got stabbed through the chest with it.
Now it has been stated that Hitsuguya has massive underlying potential - but this doesn't mean that he will be any quicker to obtain it than someone with less potential.
This does not mean that I think Hitsuguya isn't/wasn't quick in learning and developing his skills, just that I do not think that potential is something that should really be taken into account when discussing who is more talented. If you can never reach your true potential, then what does it matter what your potential was.
Now on to the question at hand:
Will Toshiro reach his true potential - maybe in a flashforward following the end of this manga
Is Toshiro currently more talented than Gin was - Not in my opinion.
Is Byakuya more talented than Gin was - As I said, Gin was hiding his powers the whole time, and still he looked on(/above in some peoples opinions) Byakuya's level, therefore I go with Gin again.
As to Byakuya vs Hitsuguya - Byakuya has always come across as more collected, smart, versatile (in different areas of shinigami skill NOT zanpaktou power). It was not (to my knowledge) stated that he got this power with anything more than hard work and dedication. I will, currently, go with Byakuya over Hitsuguya for this reason.
Therefore, in summary, my views on the order of talent for these 3 captains goes:
Gin -> Byakuya -> Hitsuguya