Like pwebb already said the Cannondale Track from the 90's is very collectible at the moment. It's sort of an anomaly. The Cinelli Laser is also a cool bike if you can get your hands on one. Good condition NJS framesets from 3renchso, Nagasawa, Samson, etc...are also very desirable.
Like most things that reach collectible status it's a combination limited availability and high demand. What makes something high demand is the trick, so just holding on to a bunch of old bikes doesn't mean they will ever be worth anything.