For me it has been:
1. Cigar box stir plate...works wonderfully well, looks sexy in my kitchen, and people look at me as if I was way smarter than I am
2.Hose / Copper CFC - Significantly shortens by brew day
3. Recirc system with topsflo pump - further reduces the brew day and works great with the CFC.
4. Heatstick - I can now do full boils in my electric stove.
5. Brew Lab - save some yeast and save some $!