I'd like a Cannondale 440MX, the Hesketh of dirt bikes. This is the bike that should have started the modern 4 stroke revolution but they were let down by problems with the EFI and ignitions....and engine unreliability and valve breakage and clutch problems and a shitload more quirky anomolies. Cannondale did their dough on the bike and its equally bad quad brother and went back to what they know best....making pushies. Despite the problems they go like a shower of shit and supposedly handle really well. You can pick 'em up for DT1 money on eBay.
ATK bought them out and ironed out all the problems and also sell an upgrade kit for the 'Dales. They also suffered from weak big ends. ATK still race them and have had some success.
The biggest problem for Cannondale was the bean counters releasing the bike before the engineers were finnished with it, I can't recall exactly if it got bad press or not but it may have and that would of helped kill it.
Some one bought a container of the ATK/dales to Aus so there are a few floating around. Had trouble selling them and was told at the time that I could get a new one in the crate for a good deal. Saw one on Ebay a few years back in Sydney. Tempted, but thought that I own enough white elephants as it is.
As for future bikes. Probably the YZ400. It's the garden variety bike that everyone had that usually becomes the most collectible. It's the hardcore enthusiast that usually goes for the exotic.
My pick, KTM 950 Super enduro. Balls out 185kg 100 hp dirt bike that never sold well and went out of production after a couple of years. I bought one new because I saw the light. Starting to get sort after now as people realise and prices are creeping.