Hmm...
1Ds
300D
5D2
1D4
My reasoning - the pro's had to be sold on the digital camera - this was the first digital camera that really outperformed 35mm film. Once there pro's were doing it, the hobbyist market would take dslr seriously which could justify the development of the 300D and bring it to the mass market.
To my mind, the 5D2 and 1D4 - even though they are both spectacular cameras - are just incremental developments.
The 1D4 video would be really cool... am always willing to learn.