This is what i used to have the most fun on, (not mine in the pic) Suzuki PE 250, Regret getting rid of it, mint ones go for big bucks now. Cant say the Duc was the "Best' bike i ever had because it wasnt it, just looked and sounded the best, sorry to offend folks.
Before my current Multi 1200 S Touring, it was my BMW R1200RT SE. Hugely capable and fun (no bike that big should be able to corner like that!) and very, very comfy.
So far I've enjoyed the past three big bikes I've had - Blade RR4, GSXR1000K6, and the R1 cross plane crank jobbie. They were all different but great in their own way. I'm hoping my new 848 Evo Corse will be the best ever though
I'd say my current Streetfight S, but previously owned bikes............it would be my Aprilia Tuono Factory. Great fun and very well made.
My current Streetfighter. Plenty of bikes are more practical, cheaper, easier to master and without the frightening mechanical clatter. But nothing else I've owned has provided so much fun, exhilaration and sense of occasion, especially since its been worked on. Its spectacularly entertaining. I never get tired of riding it and it never stops surprising me. Its even quite comfortable.