The 87 to 90 is my choice.
The 85 86 is a great  quad for playing and trail riding but the swing arm is a terrible weak point. If you want to mod it, this year has the fewest options.
The 91 92 is good as well. If you tear it down on a regular basis, the rear swing arm can be a pill until you get the hang of it. It's also the heaviest. You have a more limited set of options for mod's in the rear due to the swing arm.
The 87 to 90 has the most options for mods. 87 motor being the big difference. I have an 87 250 and 500 motor and I'm still able to find intake parts with out to much bs.