Best - Gotta be my Pace RC100, firm, sprightly, sure footed and still my dream bike.
Worst - It has to be any "Mountain Bike" produced by Kirk Precision. I had and snapped each and every version, I can't fault them for their warranty scheme.
Apologies to all contributers but the best ever has got to be the 1989 vintage MSRacing ProCompXT. Light, responsive, nimble over the gnarliest single-track. It has a delicate yet powerful nose with subtle hints of oak and ripe, freshly picked blackberries, leaving you with the tingling freshness of bathing in crushed ice.
Best of all there's still one available in the for sale section.
Worst, specialised enduro, the pressed/hydroformed alloy one, 2004 ish. Wow. Just wow. I have no idea how they managed to sell any of them.
Best, no idea, my torus was/is good, would still ride it today, except its old enough to be irreplaceable. A 4 year old carbon frame is easily replaceable.