The E30 will always be the most special M3 as it's the first.
All subsequent M3's will depreciate and not really be a collectible car. The clean and well maintained will be at a premium, and probably harder to find as these cars become more affordable. They are all special in their own way.
2008 E92 M3: Sparkling Graphite | Bamboo Beige | 6MT | CF Roof | HRE | Eisenmann | BPM Sport | Swift | Arkym | Mode Carbon | M.Perf