I would have to go with Swiftly - the later model Panther , although the Perishing which was just emerging - saw very limited service in Europe did so with an amazing degree of success , in terms of speed , hitting power and armour it was a force to be reckoned with.
The Panther showed that there was development potential within the design - there were plans to up gun it , the JP. development was highly successful and many of the teething problems had been ironed out - she had matured and come of age.
The long 75mm was able to take on any Allied or Soviet tank , she was fast stable and awell balanced package - in terms of drive - she was still weak - and for her weight she was still underpowered.
The King Tiger - too big , too heavy , underpowered , limited range , vunerable transmission , transportation and battlefield mobility presented problems.
Quality of build might also be an issue - by the end of the war materials and the proivision of quailty raw materials were making an impact.
She could destroy anything and her heavy armour provided protection - the down side came from an engine system which was under strain and "you just couldn't carry fuel to her".
In some respects the wrong tank at the wrong time.