Best Upgrade you had?

What single upgrade did you have that gave you the most impact in sound improvement? Mine would be the EMM LABS DCC2 dac and CDSD transport followed perhaps by my AUDIOMAGIC line conditioner.
Wife #2 who couldn't care less whether I want my own sound room, and doesn't care how loud it is - a Keeper!
1)Quicksilver Triode monoblocks
2)Western Electric 300B tubes
Audio Aero Capitole MK2
Without hesitation - H2O Signature monoblocks.
3 inch x-pipe with no mufflers and 3 1/2 inch chrome tips exhaust on my 04 Mustang GT.
Without a doubt the best upgrade, no performance gain, but it just sounds that damn good. Sounds like 100% american muscle baby. My K&N cold air intake kit should arrive here in the next week or so, im looking forward to that. a little bit more HP, torque and MPG

well, my gear is all pretty much mid-fi, but i would say my best upgrade so far is my denon 2900 universal player which replaced my panasonic DV-A7 DVD-A player. Better in every single aspect imaginable.

My new girlfriend is definatly a step up from my last in every category. Better front end, chassis, and rear. Good natured and a killer smile, and a lot of fun to be around. very patient with me as well. :)
Too many to pick ONE!
Bass Traps!
from an A/V receiver to SS separates. No upgrade (Except for tube separates and the switch to planars) has been as great. No even close.
Upgrades that froze me in place upon 1st listen ...
1. Lyra Clavis Evolve 99 cartridge
2. Dedicated Music room
3. 47 Labs Phonocube
I got a new wife. The highs are much sweeter now!
All these new wives and girlfriends. We must live in a mobile society. My best upgrade was an all Audio Note system correctly set up with speakers in the corners. Simply terrific!!! The best reproduction I'v ever heard. Alas, it was not to be forever. My wife still wanted a TV in the room and the only place for it was the corner. What I get now is 95% of that but as we all know the higher up you go each percentage point is worth more and more. Someday!
I'll follow Slappy's template.

Bilstein shocks & struts with Eibach Pro Kit springs dropped the car and tightened it up the way I had hoped without creating a harsh ride. Replacing my factory airbag steering wheel with the Momo Competition may have reduced my overall safety, but now I feel like I'm grabbing something substantial unlike the factory pipecleaner. Rogue Engineering short shifter has made my car so much fun to drive. And the factory BMW X-Brace made a huge improvement in initial turn-in. Not bad for a $150 part.

Introducing tubes into my hifi is one of the most positive shifts I've made in audio. I have a tube DAC and have owned different tube preamps, but never a tube amp. I'll soon be getting an Exemplar Exception II which I'm hoping will be a big step up. And I'm also considering a high powered SET like the Bel Canto SET40, which would complete my transformation into Anakin Tubewalker.

No matter how many hotties I meet around here, I just can't seem to find anyone better than the ex. Maybe I should move to the Mediterranean.
Changing the way in which my system was set up in the room and installing a projector/screen in replace of the big RPTV was the biggest upgrade. This maximized the potential of my Kharma 3.2s and my subsequent amp upgrades.

I've always thought that Italian women were the perfect package. They are so damn sexy, then in their later years- when cooking counts as much as sex- they still outperform the rest of the world!

Of course, right now I'm obsessed with Condi :)
Köchel K-300 loudspeakers, Atma-Sphere MP-1 preamp, or Audio Synthesis DAX Discrete DAC. It's a pretty tough call deciding between the three.
Losing the live in Girlfriend (of 3 years): So I could buy my Ferrari Fly Yellow Wilsons and ditch my Audi for a Honda! A couple months later a pair of Lamm's. Oatmeal and Raman Noodles allows me to use all my disposable income for stereo equipment! Oh yeah Turning the master bedroom into a dedicated listening room became possible (I don't care about sleeping in the guest bedroom!)
Early upgrades had a bigger impact; I've been working at diminishing returns in recent years. Probably my best was in 1996, replacing Rectilinear IIIs (bought in 1969) with Thiel 3.6s. The sound improvement was quite noticable. (Heh, heh, heh) Yup.
Speakers, without a doubt. Made the largest, most profound change in my system top to bottom, back and forward--the whole nine. Went from the Revel M20s (which I loved) to the Harmonic Precision Caravelles. Huge difference in $$, but the sound?! My rig is some pretty good electronics/etc. as well.....peace, warren
Building a dedicated room and switching to full-range planers (Apogee Duetta Signatures).

Smaller but still great...adding a tubed pre-amp as a mate with my SS amp.

Dedicated circuts has to be one of my smarter moves also but they were part of my dedicated room build so never had a chance to compare (before and after) results.

The most cost effective upgrade I ever made was dedicated power lines.
Ok, If we're talking cost effective: I agree with Stage.
For cost effectiveness top flight cans can truly bring out the best of your system without paying exteme dollars for speakers then spending weeks tweaking with room treatments. Also good for isolating what part of the system hums, needs improvement, ect. Followed by dedicated headphone amp with conditioned or battery power supply. IMHO of course.
The addition of a subwoofer intended primarily for HT. I've found that a few work well with movies and music. I listen to 2-channel daily with a REL Stadium III. A good sub that is intended for music will make a significant improvement to anyones listening experience. I'm hooked.
replacing my Anthem PVA2 poweramp with a CJ MF2500A has made the biggest improvement to my system,as good as the Anthem is for the money,the CJ has taken my system to a much higher level,cant wait to hear how the Supratek Chard. i have on order is going to sound!!!
Being bumped to first-class on a transatlantic flight out of London. It was dramatically quieter, and the sound emanating from the headphones was far superior to that suffered by economy class passengers using those throw-away things. I would say, that in comparison, the sound was airy, incredibly life-like and almost palpable. I felt as if I was in the clouds!

At home, the single upgrade that impressed me the most--and first opened my mind to the possibilities of better sound--was buying a pair of heavy Target speaker stands for my LS3/5As (which previously sat on a bookcase). Soon after I discovered the differences cables made. Then I replaced my Denon direct-drive with a Systemdek. Then room treatments, etc.

The speaker stands, however, remain significant. As a musician (musicians often have dreadful stereo systems) I had been highly skeptical that such changes would make a difference. And like many others, I was entirely convinced (more like opinionated, narrow-minded) that was I was hearing was all there was to hear.
In 1983 I sold my very "very high quality" SS preamp and bought a ARC Sp10II. No single component has come even close to yielding that great an improvement in my home. Next to that improvement, the next greatest improvement came in fine tuning speaker placement (down to the inch and degree) and listening position along with appropriate room treatment to control reflections.
This is 10 pages long, "just kidding." Maximum Super Tweeter
Townshend Audio extend the responce of your speaker system to 100khz. If you got a box speaker, it's a no brainer. Put it in your system for 2/3 hours of listening, and now take them out. It's truly amazing , these have a 10 point settings, I think that is a must! $$$$ Note: to many others to list.
Shunyata Hydra 8 with the Python power cord. It makes everything plugged into it come alive.
Dedicated power brought my system to a new level (cost-$600). 2nd-my Bat vk5i with nos tubes - eight 1963 and 1964 matched 7308 Amperex PQ white label w/ shield small signal tubes and two 1957 5881 Tungsol power tubes (tube cost- I try not to think about it).