In 1990 you couldn't get a 'good' ready to race mountain bike, that's why all the machine shops and custom builders sprung up.
I'm not sure that's entirely true - A Fisher Competition or Procaliber, Stumpjumper Comp, closer to home a Pace, or even one of the Raleigh USAs that John Tomac seemed to do "OK" on were perfectly adequate for racing out of the box. Bling doesn't win races, cyclists do.
I think people are far more likely to consider as retro the bikes that they were just a bit too young to get their mitts on, the bikes older brothers had or that were just way out of your price range when you were busily saving away. For me that's 1986-88 so my retro dream rides include a Tushingham B-52, a high end Muddy Fox (Monarch), a Roberts painted as a Peugeot as ridden by Baker and Gould, and a U-braked Fisher Competition (the hours I spent stroking that thing in the Brighton Bike UK store). When my daydreams turn to the road it's a Raleigh Team Professional.
The first mountain mass-produced mountain bikes enabled a lot of people to get out there and do something that hadn't been done before (or only extremely rarely). I remember the shock on people's faces when they saw a mountain biker heading along the South Downs way. I lugged (no pun intended) the thing up Helvelyn just so I could ride it around on the table top and truly say I'd been "mountain" biking. After the hurricane in 1987 I could get places that land rovers couldn't reach. All of this was new and amazing.
So, in a rather long-winded way, my vote goes for pre-1990.
Raleigh Ozark (1987)
M-Trax Ti-1000 (1995)
Dynatech Roadie (1996)