Nord Lead. The original. Advertised as 'Playing the Nord Lead is fun, fun, fun' and after having left a music store in 1995 somewhat dazed after having played one for the better part of two hours I tended to agree, so much that I started thinking about how to save up the money needed to buy one. Only four voices in the first version, an optional 8 voice expansion board also brought a PCMCIA slot so you could save more than 40 patches, but also bumped the price by 50%. The original software version 1.x is probably mostly forgotten by now, having been replaced by the 2.x software which brought it really close to the ...
Nord Lead 2. The follow-up. Fixed a few annoying omissions on the original version, most notably keyboard split and keyboard filter tracking in thirds rather than just on/off. With 16 voices at the same price point as the original version with only 4, this was a no-brainer for me and I ended up getting one new in 1997 just after they were launched, winning out over the Roland JP-8000 which was also fairly new at the time.
Nord Lead 2X. The re-vamp. Redesigned electronics, but otherwise identical to the NL2, and at a lower price point too. Well, it's also got lots more internal memory, as well as 4 more voices for a total of 20, and revised front panel graphics to fit better in with Clavia's contemporary style.
As far as I'm concerned, these machines sound identical. The only difference I've been able to notice (apart from the obvious consequences of things like the difference in filter tracking settings between the NL and NL2/2X) is that the audio output from the NL is inverted compared to the other two, so listening to sawtooth-based bass sounds in headphones sounds different. Not sure how much practical difference it would make though. The NL and NL2 sport the now-well-known "Pelle's Mode" which is some form of experimental physical modeling, intended to show what could be done with a machine using alternate software, but was never made into a product. Pelle's Mode was omitted on the 2X (spoilsports!

As for the sound itself, these machines have a solid but rather bass-unheavy sound. Altough analog in their appearence and first order design, they have a special quality which is a bit different from your run-of-the-mill analog synth.
Nord Lead 3. The Dream Machine. With 24 voices, aftertouch, ladder filter emulation, LCD display (patches can be named!), four (!) morph channels, and most parameters sporting LED rings with the actual controls being high-resolution incrementors, 2 oscillators masquerading as four, dual filters, highly configurable LFO's and modulation EG, and a unison mode which does not take up any additional voices, this would be the Nord to die for. Priced, as far as I can remember, at a price point slightly below what an expanded original NL had cost, but of course at the time the world had moved on and we were expecting more for less.
For me, the sound of this machine is a bit of a letdown. I had one until recently for about 8 months or so, and while it was great at whipping up dreamy, spacy sounds with a lot of movement, there was something about it that just didn't click with me, especially when trying to create more down to earth analog-style sounds. I think that one thing is that soundwise it is further from an analog synthesizer than the Nord Lead 1/2/2X, making it less intuitive for classic analog synthesists, despite the fact that the user interface is a total dream to work with - all parameter values are immediately visible, and for some morph channels the morphing is shown in real time using the LED rings. Not to mention the large number of modulation options - the LFO's can be used as EG's, the modulation EG can be used as an LFO, the past's the future, the future's the past, you've got it all. It even has my favorite modulation path - audio modulation of the filter by one of the oscillators.
But the NL3 also has some annoying design decisions: Unison is either off completely or sports 5 voices per key played. The unison amount can be adjusted, but with a non-configurable setting of 5 voices, the sound becomes fairly washed out and diffuse; I'd love to have a two-voice unison mode as on the NL1/2/2X. The aftertouch is more of an on-off controller than a continuous one making it less useful than it could be. And they've dropped the drum mode from the NL1/2/2X too.
The NL3 apparently did not sell well, and when the display manufacturer stopped producing the display, it was not considered worth while to redesign the PCB for another type, and the production ceased. I feel sorry for the folks at Clavia for putting so much effort and thought into this machine, yet it not being the success they'd hoped for in the market place. I think the main reason for the lack of success was that somewhere along the line they'd missed out on the bit that made the original Nord Lead 'fun' to play on. Can't put my finger exactly on what it is though. It sure has a lot going for it though, as demonstrated in this video (yep, it's me, I was aiming for a 10 minute quick demonstration but topping out at just over 50 minutes it really pushes the point that there's a lot to get your head around here once you start digging in).
As we know, the NL2X came after the NL3, and it has already been discussed above.
The Nord Wave. Well, I've never tested it at home, and since this thread is about Nord Leads, I'll leave the Wave out. Suffice to say I was hoping for this one to bring the 'fun, fun, fun' into sampling, i.e. a cross between a Roland S-10, a Korg Microsampler, and a Nord Lead, but it came out being 1/8 of what I'd hoped for: only sampling on one of the two oscillators, no sampling on the machine itself, and only two slots.
The NL4. Ah, the rebound! Bold and bouncy, it takes up where the NL2/X left off and adds a little here and a little there, and an effects unit to boot, the first time on a Nord Lead! Having tested the NL3, it's interesting to see how they've brought forth some of the more redeeming qualities of the 3 - unison without voice stealing (and more intelligent unison modes, although I still miss some configurability), the separate vibrato oscillator (but only 4..8 Hz? Come on guys, it's a synthesizer, not an electric organ from 1975!) ,better control than on the 2/2X of which slots the keyboard, mod wheel and pitch stick actually controls, and a host of other features. Instead of risking the resulting price increase as a result of expensive LED rings, there's a morph LED per knob which is still something.
The sound though is ... cutting. To me the NL4 has a very harsh sound, and while I can get some nice pads out the NL2, the NL4 tends to tear them apart. But with the NL4 it's back to the 'fun, fun, fun' bit again; one can play for hours on this machine. Still, the harshness of the sound, while apparently pleasing bands like the Freemasons and I would think Infected Mushroom, for me goes too far in the competition for space in the mix. The NL4 is like a tiger, clawing it's way through the speaker cones. Which is certainly what some people want. Not really sure if it illustrates my point, but here's a demo which fails to produce the string pad I wanted (the drums are the only non-NL4 sounds on this track).
A year after the NL4 and we're spoilt for choice with the NL A1. To me, this machine is the most analog sounding of any Nord. Tweaking the filter as can be heard a couple of minutes into this demo (after the various unison modes and chorus have been demonstrated on the introductory pad sound), sound increadibly analog, and it's easy to imagine oneself standing in front of a Prophet rather than a Nord. The effects unit is more intelligently made up than on the NL4 in that both reverb and delay are possible at the same time.
But to my mind there are some annoying omissions. This is one of the few synthesizers to sport ADR envelopes with no sustain parameter. After my initial reaction that 'yeah, most sounds have a sustain level of 100% or 0 anyway', I started to miss the sustain parameter. The two oscillators are invariably linked through a dual oscillator configuration which I didn't find made things easier at all, and I again found myself missing things like being able to set up a pulse and sawtooth wave one octave apart with detune. Apparently the oscillator section has been completely redesigned since earlier machines, which might attribute to a lot of the analog sound - with both oscillators being generated by a single signal processing model, some of the deficiencies of more conventional DSP based oscillators can be avoided.
The ADR thing apparently has bitten Clavia already - in legato (mono) mode, the amplifier envelope actually has a (albeit fixed) sustain level, because otherwise the sound would eventually die out if never retriggered (touché!

So, confusing to program, and lacking some of the basics drops the A1 down a few points. Plus it doesn't have a distinctive character like the other Nords do.
What is the best Nord then? Well, I think first it comes down to what you want soundwise: do you want a really analog sounding machine? Then go for the A1, no question. Do you want a machine which picks up where analog synthesis leaves off, and complements other machines in your rig? Then go for the 2 or 2X. Something more agressive, to compete with that guitarist with his 42 guitar pedals and knobs-to-11 amplifier? Go for the 4! Like to wander about all day in an ethereal landscape without setting your foot on anything more than remotely feeling like an analog filter sweep? Go for the 3!
Personally, I like the 2/2x best. It's got character, a solid core to the sound, and pays heritage to the original Nord Lead. The 2 gives us Pelle's Mode for the days when we're feeling really adventurous, but the 2X gives us many more patches (and a couple of more voices). Ah, choices!
If I had enough money left over after getting a 2X, I'd get an A1 to get two machines which would complement each other rather nicely. But the A1 would face stiff competition soundwise from a true analog machine such as a Prophet (take your pick) and it is harder to justify specifically getting a Nord.