My 2000 RM250 2-stroke.
Bought as a non-runner. Got it running the first time and the motor sounded exactly like a watch in a blender.
I put way too much money into that bike but now it's nearly a brand new machine. It's a finicky lil' bitch and the carb will most likely never be dialed in. But its one of those bikes that challenged me and perplexed me so much I can never part with it. I have spent countless hours sitting on the stool in my garage trying to out wit that bike. And I even think once in a while she lets me feel like I'm winning.
Also, my 2001 F350 Dually. I know it's not a bike but I thought it was worth mentioning. 10+ years and 200k and we're not anywhere near finished with each other.