What do you feel is the best ham radio improvement you've done either inside or outside your shack?

For me so far its a headset, it makes operating so much easier with 2 hands.

My next items on the list are a radio mod that will give me a panadapter on my 450d, and a better antenna up higher.

I'm hoping that the antenna will let me talk to all the stations I'm hearing now, but can't make it out to, I seem to have a dead spot in northern Europe.

I'm a headset guy but that's more a hearing solution for me. The first best thing will always be an antenna improvement whether it's higher, bigger, longer or whatever.

Outside the shack: got my antenna up HIGH. 65ft, give or take, according to my plastic protractor theodolite Obviously a shit hot antenna is always going to be #1. Ideally everyone should simply put in a 100ft tower with a SteppIR or some other monster 40M and up Yagi or the like. If everyone did that we'd have nary a discussion about propagation or QRP. But, failing that, you must get your antenna HIGH, consider 50ft an absolute minimum. Buy/build/beg/borrow/steal, but get it up HIGH.

Inside the shack: bought some shit hot speakers. JBL LSR305 self-powered studio monitors. Blew $250 for a pair off of eBay. BEST $250 I've ever spent on amateur radio, I shit you not. If everyone used these speakers and bought any one of the top 20 radios on the Sherwood Engineering rankings we'd have nary a discussion about receiver performance. They work great for music, too

Honorable mention: bought one of the top 20 radios on the Sherwood Engineering rankings. Any of these top 20 will make any of the lesser radios seem deaf as a post. I had an IC-7000. Couldn't understand why I couldn't hear anything. Bought a Flex-3000. Could not believe the difference, it was that dramatic.

Starting out in HF? Just go buy an IC-7300 [/mic drop] It's in the top 20. After that, when you start to move up in the world, you can choose another from the top 20, but IMHO nobody should start with less than an IC-7300 unless they are absolutely hand to mouth but still desperate to get in the hobby (no disrespect meant to anyone who is struggling for money, life is not easy). Used 7300s are plentiful and south of $1K.

Having more than one antenna for each band.

Not that I have antennas for each band, but I have more than one HF antenna and more than one VHF/UHF antenna.

I don't have any kind of spectacular antenna farm, just wires. But, it is very nice to switch between them and it often makes a huge difference in signal strength on receive.

FWIW: all my radios are in the top 20 of Sherwood's ratings and my main radio is in the top 5. I am also using those same speakers. I have an audio mixer with all my radios and my PC going into the mixer. Makes it really nice to have one set of GOOD speakers for everything and be able to hear more than one thing at a time while being able to adjust the levels of each device. Same goes for headphones which plug into the mixer.

Top pick has to be replacing the wonky FT-707 with a TS-590s (night & day difference!).  Close second is the ladder fed FWL's hung from the trees around the house (160m & 80m) ...

Putting two utility poles in the back yard. Now I don't worry about branches breaking or trees falling and bringing down my antenna.

They cost about what a mid-grade amplifier would have set me back.

Inside the shack........a nice, comfy chair.

Building a shelf for the desk to help organize all the stuff, helped too.

Heil Pro Set ic headset and a Treadlite footswitch for PTT. Hands free operation.

Second best would be my 6ft tall rack cabinet on casters to house equipment.

Just moved an old server rack into the man cave/radio room. All the IT gear is mounted, now to mount the radio gear.

Outside ... getting a backyard and might be soon

Inside ... HF LDMOS amp but only after the backyard

With two arfhams chiming in about racks, I feel compelled to show mine

It's not the "best improvement" though, although it was a pretty nice one, and one I am quite proud of. It lives in the basement. I operate everything from my PC upstairs.

Front plexiglass door removed for clarity...

Now I really want to replace my 50A power supply with a rack mount.

I got a 60 ft Rhon-25 tower free for the take down.
Placed it at the halfway point between north peak of house and south peak of barn.
10-15-20 meter Mosley on top aimed towards northern Europe.
160-80M fan dipole below the Mosley.
40M dipole at 90 degrees from Mosley below the 160-80M fan.
A solid common ground between tower, house, shack, phones cable etc....
I have an amp but it's not in line... never felt I needed it.
Have a couple decent speakers, fair set of cans.
Dats 'bout it... works well for me..

For me it was the 43' vertical and the base mounted tuner. I think I've mentioned before how much
switching to a vertical has changed things for me.

Prior to it, all my antennas had been roof-mounted horizontal wires (along with a few experiments.)
In my part of AZ there's no trees, so I was looking at dipoles or end-fed wires 12-16 feet off the
ground over really crappy soil. Pretty much me entire ham career I'd been told "dipole dipole dipole."

Then I got a buddie stick kit to play with, and the buddie stick clamped to a CMU block on the ground
was outperforming my wire by a wide margin. That lead to me expanding it using the military whips,
and 80M coil and ground radials, which was better still and that justified the 43' installation.

For me it was like getting glasses and seeing leaves for the first time. I could finally hear with the 43'.