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.
Current: 2008 BMW M3 Sparkling Graphite/Bamboo Beige/6MT. 1999 BMW M3 Techno Violet/Dove/5MT. 2007 BMW Z4MR Alpine White/Imola Red/6MT