You could also argue that the last "real" M.G. was built before WWII, and it was designed by Cecil Kimber.
Or, you could say the last one was the TC before they went to those silly little modern wheels.
Or, perhaps the TF before they put a trendy streamlined body on a TD chassis to make the MGA prototype.
Or, in 1967 just before they did away with tradition and put foam padding on the dash and smog equipment in the engine.
There are a million ways to look at it, but my feeling is that the essence of the real (early) MG sports cars died with the introduction of (excluding saloon cars) the MGB. MGA was certainly a departure from the usual pure-bred sports car but it was still a basic open roadster so I give it some credit. The MGB was far over the line between pure sports car and daily driver and while it is a great car offering good performance, it was not designed with the same ideals as the company's early high performance offerings. The later MGBs were even farther from this, becomming more of a "sporty car" than sports car.
With the original M.G. Car Company going out of business in 1980, thus ended the era of the "real" MGs altogether. Anything made after that is not a product of the original M.G., but instead a product of a different company that paid for the use of the classic MG badge. Yes, they are MGs because whoever legally owns the name is MG, but they are not the same MG as the company that started the marque.
If you really want to get down to it, the last of the pure M.G. cars was probably the TC. It was the end of an era and anything after is typically considered a "modern" M.G.
Now before I get flamed, I own and love an MGA and two MGBs so my comments are not meant to be derogatory. The MGB is a fantastic car! It is just not as pure a sports car as the models built 40 years earlier. And the MGA offers a driving experience unlike any other - one I would not trade for anything.