Pzkpfw. Panther Ausf. F
Upgraded Panthers with better armor, Schmalturm with the 88mm KwK 43 L/71, heavier armor and yet lower weight IIRC, built-in stereoscopic rangefinder, ...
Yet, all protoypes were destroyed by the allied during target practises. One survived i think, but it has more parts replaced than it had of it's own.
Panther II is also intresting, but i think the Ausf. F would have been better since i guess the costs and mobility/engine might have been an issue on the Panther II, while the advantages were less. It had strongly improved armor to 10cm angled frontal armor, heavier side and top armor, heavier Tiger II turret etc.
Luckily, one of these survived the war and target practises, and you can still visit it at the Patton Cavalry and Armor Museum. Sadly, they put a Panther Ausf. G turret on it.