The three things that revolutionised my brewing....'cause i can't just pick one.
Each one of these radically changed the quality, and ease of brewdays..
1. Grain mill.
Without it, i would still have to order on line, long distance recipe by recipe to get my grain. Closest decent shop to me (as far as i know) would be Grain and Grape, BrewAdelaide, or Beerbelly.
Currently giving my business to Beerbelly as i have family in Adelaide and am up there fairly regularly, and Amanda has always given my exceptional service.
Having a grain mill allows me to buy in bulk when i travel back to Adelaide, saving me a shitload of coin in the long run.
2. Temp control
Without it, my beers were a bit hit and miss. Nowadays, strict control over temps gives my consistent results, repeatable every time.
3. Refractometer
Allows me to keep an eye on, and track my boil off vs gravity giving me an idea of how i'm going on brewday. Love the small sample, can take a reading any time during the boil to guage progress.
Also use it (with a conversion chart or software) to check specific gravity. Haven't owned a hydrometer for several years now.
These three things literally changed my brewing experience for the better. All of the other brewing acquisitions i have work arounds for if they fail, but without these, i'm pretty much rooted.