I missed this thread previously, so I'll throw in now:
Juno-106: This was my first synth. What can one say that hasn't already been said? Excellent pads, strings, harpsichord-type things, bells, frogs
, and can do certain impressive effects -- I have a killer air-raid siren patch on mine. That lovely Roland OTA filter makes a great counterpoint to the typical Moog transistor ladder. Yes, the unison mode is a botch -- all of the DCOs phase-lock and everything comes out sounding the same. And there's the infamous 80017A issue. Mine has a dead one on voice #4 right now. Surprisingly complete MIDI implementation for the time, although you don't want to use a 106 as a master keyboard because it has some quirks. (You wouldn't want to anyway since it doesn't have velocity or aftertouch.) Very versatile for a one-osc synth.
Matrix-6R / Matrix-1000: I have both of these, and they are very similar. A more cutting sound than the 106, and it has more versatility in both audio and control generation and routing. You can get pretty wild with it. A patch editor is recommended for the 6R and mandatory for the 1000 (which has no editing controls). As someone else mentioned, the 6R uses two DCO master clocks while the 1000 only has one, so the 6R sounds thicker on certain patches. The 6R has some MIDI issues (mainly effecting the functionality of patch editors) which were fixed on the 1000. Patches are interchangeable, although the 1000 will "forget" patch names and replace them with default names. Some 1000 owners complain about the transformer humming, but mine doesn't do that.
Poly-800: Spent a couple of hours with one when a music store was closing out the model and they wanted to sell me their floor demonstrator for cheap. I wasn't that impressed; I didn't like the paraphonic filter and that "triangle" waveform was pretty bogus. (Caveat: The floor unit was rather beat up, so it's possible that it was not working quite right.)
JD-800: Not a VA despite its appearance and voice architecture; it's unapologetically digital. Absolutely wonderful interface; you do need to take a bit of time to learn how the editing modes work, but once you figure it out, it's very quick to work with. (And it can serve as a programmer for a JD-990.) Sound is very versatile, with four layers and about 120 waveforms in its wavetable, including some uncommon things like attack transients -- snaps, pops, clicks, etc. I actually like that the digital filter does not self-oscillate; you can crank it into extreme resonance settings without any self-oscillation tone interfering. If all four layers are on, it's only 6-voice polyphonic. Effects work completely differently in single and multitimbral mode; a patch with effects won't sound the same in multi mode. The dreaded "red plauge" has gotten to mine, so some of the notes on the keyboard no longer function; that's OK since I mostly play it through MIDI.
Fizmo: Great implementation of the Ensoniq Transwave concept, with extensive wave tables that allow you to build impressive evolving sounds. Unfortunately, the user interface was botched; there's only a single 4-digit display to present all information, and some parameters are not editable without a patch editor (and the only patch editor available is a hacked version of Soundiver that only runs on older Windows systems). I wasn't that impressed with the built-in vocoder; it doesn't seem to have very many channels. The MIDI implementation is an enormous mess; it will play all right, but good luck saving and restoring patches, and read the manual carefully. By now, nearly all of the remaining ones have had the deadly voltage regulator problem fixed; I fixed mine when I bought it. Panel graphics are an acquired taste.
V-Synth: Incredibly versatile synth that I think has been under-appreciated and under-explored. A combination sampler/VA with some routing flexibility, and a bevy of filters and effects, but still a subtractive architecture, so it's not totally coming out of left field. The array of performance controls is also under-appreciated: the Time Trip pad, the D-beam, the two soft knobs, velocity and aftertouch sensitive keyboard, and the best implementation of the Roland pitch/mod T-handle that I've come across. You can load and save samples via the USB interface, although you'll need an older WIndows or Mac machine to do it with, since Roland has not updated the USB drivers.
K5m: This one is
out of left field. An additive synth with 128 partials, divisible into two groups of 64. Additive is so versatile and can create such out-of-this-world stuff, but you do have to spend a fair amount of time experimenting and learning it. Excels at percussive sounds, bells and gongs, as well as woodwind and reed type sounds and plucked-string sounds. Filter is lame but you don't need it. The LCD graphical display was very advanced for its time, but it's still cryptic and you have to spend some time reading the manual and practicing with it to get the hang of it. The backlight was dead on mine, which is typical; I fixed it.
Solaris: This has become my "do everything" synth. It's practically like having a polyphonic modular, and one with a huge variety of modules at that. Implements a bunch of completely different synthesis methods: VA, wavetable, sample playback, FM, AM. The sound can be as warm or as cold as you want it, depending on how you patch and what effects you use. The user interface with the display-and-knobs sets for each functional area is a joy to work with, and you can get results fast. One thing that's a bit odd is that everything including the operating system is stored on a flash memory card that is plugged into the slot in the back; there is no internal memory. To load samples or back up patches, you need to take the card out and put it into a card reader attached to a computer.