Wow...

Going against the grain here...

Best:
My Mackie Onyx 400F I bought the CEntrance driver which solved the only issue I had. Mackie has recently released a very stable and complete update of the drivers. I am using the new drivers and it is great.

Sonar 6PE... my favorite! (from 2xl on)
ADL600 tube preamp. The most amazing preamp I've ever heard.
Neumann TLM49 A vocal mic for the ages.
My ASAT Z3 and ASAT Classic.... Leo to the bone!
Voxengo!!! The best VST plugs on the planet. He could sell them for much more then he does. In fact, I think the low price misleads some folks. They are incredibly deep programs.

While not the best ever, I did purchase Autotune 5. Quite frankly, it works without audible artifact. You simply can not tell it is working other than the vocal is in tune. A tremendous improvement from previous versions.


Worst...
My ADATS. I bought them new for around $7k. The next month, they were about 1/2 that price. They worked good but broke down about the same time Alesis went bankrupt. They sat in a repair shop in Dallas for almost a year because no parts were available. After they were finally fixed, I used them a couple of months and then went to the HD24 system.
I gave both of the ADATS away last month.

best; Waldorf Edition, Musicman Stingray, Sennheiser ME40/K3U SDC, Sonar 5/6 PE, Epiphone Valve Jr. head, TC Powercore,

Worst; Lexicon Core 2, Behringer ADA8000, any solidstate Ampeg, Cad dynamics, shure Beta 57, MOTU microlite parallel

Best : Mackie Control Universal, Mackie Onyx 400F (with the new drivers recently released), PodXt Pro, EzDrummer, Sonar, any of my Dean guitars, Washburn N4 guitar

Worst : Korg D1200 (hate the fact that Korg's multitrackers only have half the track-count when going 24bit), a couple of Gibson-guitars, Marshall Valvestate (both generations owned), Behringer mixer of some sort, Behringer Enhancer thing, a Peugeot 206 without aircon, DFH Superior

Stuff that's Ok, but not great, but not that bad bad either : Esi nEar06, AKG K240S, Maverick Custom F1 guitar

Best- My Brian Moore
Worst-toss up ART SGE or AKG C3000
Rj

I'm embarrassed about some of the bad decisions I've made, but here's my list: 

Best:  
Vocalist Workstation EX - I still love it. 
Studiologic SL880 - I love having 88 piano keys to play on. 
Mackie1402-VLZ - It was like hearing clean sounds for the first time. I only wish I would have spent more for the 1602. 
M-Audio Delta 44 - Old, yet still going strong. Glad M-Audio decided to make Windows 7 64-bit drivers for it. 

Worst:
Here's a story I've been wanting to get off my chest: "Roland Music Processing System (MPS)" was a very early DOS music sequencer. It wasn't too bad for those days, but then Roland announced a new product, "Mesa"). Naturally I wanted to upgrade to the new version, however, Roland would not give an upgrade price. You had to pay for it new even though I had the previous product. Back then (circa 1987) software was expensive and I didn't have a lot of money. I wrote them complaining that I should get an upgrade price on their new product since I already had MPS and they refused. I swore at that point that I would never buy any products from Roland.... and I haven't. I've had Moog, Ensoniq, Yamaha, Korg, Casio, Alesis, etc., but I would never even consider buying anything from Roland. I was already using Cakewalk when Roland bought Twelve Tone Music otherwise I would have never bought Cakewalk or Sonar. So there you have it. To make matters worse, I wound up buying Personal Composer and not long after I bought it, Jim Miller, the guy who developed it who lived in Hawaii stepped on a sea urchin and died. Following that, my brother who worked at a software business gave me a demo version of Cakewalk and I liked it and bought the full version and have been using and upgrading it ever since. BTW, "Mesa" failed miserably. Serves them right! 

DKI Synergy - Paid the price big time for being on the bleeding edge. I spent months writing BASIC software to try to control it with some success. 
Yamaha MD4 - a 4 track cassette recorder - nothing sounded good on it.  Sold it on eBay.
Yamaha TX81Z - It was a poor man's rack mount DX7, but I never really liked the sound of FM instruments and programming it was a pain. 
Elka String ensemble - Wouldn't stay in tune and in general sounded like crud.         
Wurlitzer Electric Piano - I know some people love these and many softsynths emulate the sound, but I think it sounded and played like crap.
A Getzen piccolo trumpet that never lived up to my expectations. 

Runners up: 
I had an old Micromoog I should have never sold since it was my first synth. Wish I still had it although I have a free Minimoog softsynth that sounds better.  I miss being able to twiddle the knobs on it. :)

Best- Beer.
Worst- cigarettes.

Worst - Digitech Midi Vocalist. Truly awful unit which is meant to produce automatic harmony vocals

Really? I have the Vocalist Workstation EX and I love it. I don't know about the automatic harmony, but I send MIDI tracks with the harmony notes to it and I like the results. To each his own, I guess. 

Best: Les Paul Custom, LP Special and LP Junior (all bought second hand), Alesis A6 Andromeda, Roland JP-8080, SONAR, Yamaha MG166c mixer Worst: Behringer XENYX 2442 mixer. Bought to replace a dying Yamaha. A couple knobs didn't work out of the box but could work around them. Then Channels 1 and 3 stopped working. Replaced by above Mentioned Yamaha. I'll never buy another Behringer product again.

Simply timeless. Best= Yamaha NMS 10's Worst= Roland MP32 ( cannot even give it away and it cost me $1,500 )

A different tact here, I am approching this from the indirect value I got from the purchases rather than the quality of the products.

Best - probably the Steven Slate EX $20 no brainer from Audio Midi, why, because it got me a crossgrade to Kontakt in an NI sale. If i had not got that no brainer I probably would not have got Kontakt which is now my most used VSTi.

Worst - X1, and before the row starts, not because its a bad product, but because of inertia on my part,I have not got round to learning it, so I might as well have waited for X2. X2 looks like it will be very nice with some features I want. Looks like they have not messed further with the interface so I am going to have to take that inital pain.

Not read this whole thread but I'd like to chime in...

Best: Omnisphere, Stylus, Trillian.  Just amazing for the value.  I'm shocked these are as cheap as they are for as much as they do, constantly hitting us with free updates, they work flawlessly, never crash.  I can't imagine anything that's close to these or that I find more useful.  Melodyne gets honorable mention.

Worst: I buy most every sample library I can, I think I have at least 500+ libraries.  There are a ton of duds in there.  Maybe 25% of them I've found to be completely useless for one reason or another.... either bad production, useless timings, completely disorganized messes, etc.  I figure the duds are just part of finding the good ones.  Anyway I don't have time to make a complete list but I always welcome messages asking about a specific library, I'd be happy to give anyone my personal feedback about a library before they buy something.  You could send me a PM.