The Revell AG 1/72 Leopard 2s (Leopard 2A6, Swedish Leopard 2, Leopard 2A5/A5NL). These kits have defeated four attempts to build them. The problem is that you have to glue the suspension arms to the hull, the wheels to the arms, then bend two lengths of hard plastic track around the sprocket and the idler.
1st attempt Leopard 2A6: didn't work too badly, except that one of the idlers broke off and I had to pin it back on. It was only after I finished that I realized that I'd bent the arms down, and the tank was now too low (All. my. friends. wanna be a lowrider...)
2nd attempt (2A5/A5NL) Three suspension arms broke off, and the track broke in two places because I didn't let it soak long enough in warm water.
3rd attempt (Swedish): I used a hairdryer (Acme Corporation, I think) to warm up the track. Unfortunately it warmed up the suspension arms, too, and they pointed in all directions.
4th attempt (2A6) This time there was going to be no guesswork. After setting the suspension, I cut pieces of .040 plastic and glued the tops of the wheels to the bottom of the sponsons, which allowed me to press down on them. It worked. Of course I couldn't do the complete run, but as they say, sideskirts hide all sins. Sadly, after I'd primed it I realized that the rear end was jacked up.
5th attempt (Border Models Leopard 2A6): Shake and bake kit gave me a perfect model after two days of building. Expensive, but after all I'd spent on the Revell kits... The end.