I just built the crane B model for 8094, so thought I would bump this thread. A few B models for the largest sets were amazing but most were forgettable. I can see why they stopped making them after the growth of Rebrickable and custom moc instructions being much more common now. Many of them were good but felt like inferior versions of the main model, so there wasn't much reason to keep it built. I often built the B model several years later with spare parts not from the same set.
My favorite ones are in 8868, 8480, 9396, 8094, 8485, 8425 and 8458. I especially like the ones where they introduced new and very different functionality like 8868 or 8485. The Universal sets also had good ones but sometimes had no clear main model to begin with.