There are any number of speakers that could be touted as KEF's best ever.
I do agree there was a change for KEF around the turn of the century, good and bad. They released the KHT range, starting with the 2005, and while sub/sat systems weren't a new idea, pretty much kick started the whole lifestyle speaker market. This was one of the major turning points for KEF. Around the same time, there were a couple of ranges I wasn't too personally keen on. I never got on with the original XQ series, finding them too soft and warm, and really didn't like the top mounted 'hypertweeter'. Following on from the awesome Reference range which included the Model 4.2's, KEF radically revamped their Reference series and did away with the coupled cavity, force cancelling internal bass drivers for a more conventional design, but with sculpted cabinets. This new sound wasn't as warm and bass rich as the previous ones, and many didn't like the change, including myself, but as with the previous series, the following Mark II Reference provided a huge step forward, with major changes, but unfortunately with a major price increase as well. The older Reference may have had more bass, but the more recent Reference has a tighter, more accurate bass. Despite this change, and the price increase needed to implement changes to improve the early 200 series, it hasn't stopped the Reference range being one of our most successful speaker ranges in recent years.
I originally got into KEF because of the technology behind them, and fell in love with the 104/2's in the very early 90's. The coupled cavity, force cancelling bass drivers provided the most potent bass I've ever heard from a cabinet of that size, and the d'Appolito HF/MF arrangement on the front of the cabinet producing great imaging, without the issues many speakers had because of a single HF and single MF arrangement - you could say that the d'Appolito arrangement could be seen as the nearest thing to the UniQ idea with conventional drivers. Since then, I've gone on to own 1.2's, 4.2's, 104/2's, 105/3's, 201/2's, 207/2's, and now the Blades, which are far and away THE best speaker I've ever heard in my life. Again, this was another speaker that drew me in because of the technology behind it, and once that is understood, it's then possible to appreciate the 'cabinet free' music that is produced in a totally effortless manner.
So although the 104/2 and the Model 4.2 will always be special to me (along with the KEF based Jim Rogers JR149 loudspeaker from the 70's), the Blade has to be the best ever KEF loudspeaker. Im sure if Raymond Cooke were alive today, he'd be extremely proud of what the design team have done in producing one of the most stunning sounding (as well as looking) loudspeakers ever built.
Any company that is always trying to push boundaries and try new things will always have a patchy history. To never produce a duff model or range would be a complete fluke. The Q series was quite an achievement, and the new R series is a phenomenal achievement (as you're all about to find out), and with other products coming, I think the coming 12 months - the year of KEF's 50th anniversary - are going to belong to KEF.
But that's just my personal opinion