My short list:
XR650L (DR650, KLR650) budget exploring
Yamaha GTS1000 Not just for the unique bike but also to give lessons in electronics
KTM 950R Super Enduro: not just a Adventure but a 100HP dirtbike
KX100 (as a youth) so you know what smokers are...
CR500 - KX500 (adult) lesson in humility
Yamaha R1 (bmw1000rr, GSXR, etc) so you now know what fast is...
Some type of cruiser / bagger / tourer to realize what you miss about riding the above bikes.