What Is The One Thing To Do To Improve Your Coffee?
You're expecting me to say 'home roast'. But there's one thing more important - a grinder that grinds consistently. To get that consistency you'll need a burr grinder. I use two - a manual one that's great for travelling (and home use) and an electric one.
For home use, I use the Krups GVX231.
Whatever coffee brewing method you use, grind size consistency will improve the end product. For espresso, it's critical. The GVX231 will grind from coarse - for French press/cafetiere, through filter and Aeropress to stove top moka pot, espresso and through to Greek coffee.
Some competitors in the coffee world championships then use two fine sieves to further refine the grain size.
Once you've got your grind size correct for your brewing method, experiment how coarse you can make it and still get the same result. You may find, as I did, that the optimal grind size varies with the coffee you're using.
When you home roast and grind, you're controlling most of the factors that govern whether or not you'll be able to make a good coffee.