I'm guessing, like everyone else, but based on other car models:
'84 'cause they're the first,
'88 'cause they're the last,
'85 V6 'cause they had the first V6,
'86 1/2 fastback 'cause they had the first fastback.
Which leaves '87... Can't think of anything special about the '87... which means they'll probably be the most collectible.
In general, go for low miles, original equipment, no damage.
That said, I'd love a 5 or 6 speed manual transmission Fiero with a V8 or 3800SC, maybe with a Ferrari or Lamborghini body, and HUD, maybe a convertible or how 'bout a ...