Easiest workflow to get better audio when using a DSLR
Imagine using a point-and-shoot camera where you point the camera at something but instead of actually looking through the viewfinder, you only pay attention to whether or not you see an exposure warning light.  You don't validate focus.  You don't validate composition.  You only validate that the sensor is receiving signal not blocked by the lens cap.  That is kinda how people treat audio, whether using internal microphones or shotguns strapped on to their cameras.  When they listen, rather than just looking at the meters, they are still mostly validating that signal is passing through the microphone.

What you can learn by listening to the microphone as carefully as you might look at framing, composition, balance, focus, perspective, etc., is that many audio environments are a mess.  Many audio environments mess up sounds that can only be recognized/understood because we have a really good stereo imaging system (our ears) and a very powerful noise rejection / signal enhancement system (our brain).  Randomly locating a microphone in such an environment can sufficiently compromise the signal that our ears and our brain cannot properly (or will not happily) decode it.  The solution to that problem is to place the microphone where it is not overwhelmed with flutter echo, modal resonances and cancellations, etc.  Usually, that means placing the microphone 3x closer to the source than to any source of confusion, which means keeping it within 6"-36" of the source source (depending on microphone type, source type, room behavior, etc).  The reason that pro sound guys boom their mics is because there is absolutely no substitute for putting the microphone in the right place.  The reason they use $1000 microphones is because if you are going to go to the trouble of getting good sound, you might as well get excellent sound.

There is essentially no microphone you can buy and attach to your camera that will give you good sound in a variety of circumstances.  There are many mics you can buy and attach to your camera that will give you good sound in one particular circumstance: your talent is approximately 2' in front of the camera, and there's no terrible acoustically reflective surface too close to either the talent or the camera.

DSLRs often add insult to injury by also having poor quality preamps that compromise both good and poor microphone outputs.  But the most important factor in audio quality is location, then an appropriate microphone type (omni, cardiod, super-cardiod, figure-8, etc) for the location, then the quality of implementation of that microphone, then the quality of implementation of the preamp and finally the digital recording format.