Is there a reason the 10 mil needs to be made in an hour? do you have a 10 mil bounty on your head and the hunters will arrive to collect in just over an hour?
Everything in ED stands to make you money in one form or another. The time it takes also can vary.
In the past Bounty Hunting was the fastest way to earn credits, then changes come along and progressively one game loop gets faster at making money then the next. Its a inflating issue of ED. But in the end if the goal is money divided by time then fun is not the important factor.
I understand the question is what is the fastest way to make money doing what i like, but if you compare it to other game loops something will lose out, Money or fun. @Bryan K is correct as far as i know, Combat Zones are likely the fastest way to earn large sums of credits assuming you can kill 40+ ships in under an hour.
Most who have been down the path of following the fastest money route will likely agree in saying, it's better to play what is fun then following a grind that you don't really enjoy just to make quick money. But that is just a rant from an old salt who wishes they spent more time actually doing things during his progression that where fun then trying to break the 1 billion mark, or grind out ranks. It can backfire on you and cause a burn out if not careful.
You can stack massacre missions from different mission givers. If you're allied with all those factions, that's 20m-40m per mission. Stack pirate assassination missions of the same enemy faction as well (1m-2.5m per mission). That's easily 10m per hour. Obviously, there's faster ways to make credits, but for me the credits are side benefit of my primary goals.