For me the most important process improvement was patience. The only time I'm even in the same room as my fermenters between pitching and bottling is if I'm adding dry hops. Otherwise I just let the yeast do their thing and come back in 2-3 weeks to claim my booty.
As far as equipment goes the most valuable piece for me was my grain mill. Prior to that my efficiency was all over the place from high 60's to low 80's depending on who milled my grain. Not I'm pretty much solidly in the 84-86% range.
The second most useful piece of new equipment is actually a silicone oven mitt. I BIAB, and this lets me squeeze the snot out of my bag (without burning my hands) to get most of the wort out of the grains. In conjunction with my grain mill this has really helped me hit a steady efficiency, because I can get both a consistent crush and a consistent squeeze.
Least valuable for me has been whirlfloc/irish moss. I stopped using it about a dozen batches ago and haven't noticed any difference in my finished beer by the time I'm ready to start drinking it.
One of the most important things is the mop and bucket.
For me it's my "brewery towel" - same idea.