best model year for a particular brand?

I'd agree on the early 90's Zolatone era for the Marins.

Around 1997 for a lot of manufacturers. By this time most had been doing MTB s for a decade and had things squared away. Raleigh were doing some surprisingly decent bikes during this period, largely overlooked as not sexy enough despite being as good and capable as any Fisher. Saracen had refined their frames and by this time had tidied up the seat stay-seat tube-top tube design to one what was neater, lighter and stronger. By this point even their low range frames were tidy, well thought out designs.

There will be exceptions, but for the most part the period around 95-97 is the point at which most manufacturers refined their original designs to a point of maximum efficiency, the ultimate evolution of the rigid. The downside of this is that a lot of the engineering solutions were the same throughout, so while in terms of design and performance this era may be the pinnacle of retrobikedom it is perhaps also the point at which a lot of the charm and character was at its lowest ebb.

A modern for Fucus is liable to be a vastly betterp machine in every way, but a tired and leaky old frogeye Sprite is still liable to make you smile more, and that's where MTBing was at in development terms.

For me the game changer was suspension. Until about 93 (?) the design focus was on top class steel/ti rigid frames at the top of the range. From then on the focus was on suspension which means design at the top end moved away from steel by and large. So if we're thinking about the classic idea of a retro mtb being steel (not everyone's cup of tea I know), for top end bikes, post 94(?) is more about the start of the suspension take-over (as it should be in terms of evolution of the sport) than the refinement of rigid steel. Having said all that I wasn't involved at the time - this is just my observations from being involved in recent years. Happy to be put right by Chopper or anyone else!