I have spent MANY hours doing actual tests to see which hero can actually backdoor a tower the fastest, while using a stopwatch to record the actual time it takes to backdoor. I have tried every viable hero along with different build configurations for each one and hands down the hero that can take down a tower the quickest is Alchemist, which is funny that he's not even mentioned in the list posted by someone else as people that are best at it. In fact Alchemist came in over 2 seconds quicker than the 2nd fastest hero with the best possible item configuration. For item configurations, I even tested combinations of items that you would never possibly get, although with alchemist his build is very viable nearly every game that doesn't end too early. Some of the other heroes I tested with item builds to get to 2-4 seconds longer than Alchemist had such ridiculous builds just to achieve that, it would never happen in a real match. The reason Alchemist is able to backdoor so much quicker than any other hero is because of his BAT being so much lower than any other hero, it allows him to hit towers much quicker than anyone else. I know most of you will say a hero like Troll Warlord hits faster, but keep in mind that Alchemist having the lowest BAT in the game means that nobody can ever hit as fast as him since there is an upper cap of how many times a hero can possibly hit in a second, and with Alchemist that is 5 hits per second. So even though his attack damage may not be as high as some other heroes, the fact that a tower can only regen 90 health per second and he can easily do over 5x's that amount of damage per second due to hit BAT with armor reduction, he wipes Tier4 towers out in 4.92 seconds with backdoor protection in place. If anyone is able to do a test where they can beat this time, I would like to know what hero you used and your item configuration because every other hero tested with ANY items, came in at slightly over 7 seconds at the lowest.
For anyone who responds, please do not give situational I think this would work feedback that you think would trump what I have already specifically tested. I would want real world tests done to prove this can be done any quicker than this by a single hero.
If anyone is curious about the build configuration I used, I could give the build if anyone is interested. It's viable for alchemist to achieve this build too since he can accumulate more gold than almost anyone else in the game, so no items are out of his reach.