I don't think that question can be answered simply.
Are you looking to spend lots of money initially? Are you looking to spend lots of money in repairs? Are you looking for rarity? Are you looking for a piece to sit under glass and look at while you sip chardonnay from afar?
Most people buy cars based on what's in their price range and what they want. The body style of the Supra change
very distintively throughout it's lifetime.
E.g.
MkII:
MkIII:
MkIV:
Old Supras are in poorer states generally and require much more work but are often easier to attain, while the MkIV is far more expensive and fewer were made and they are usually beyond expensive ($25,000 USD++ for the desireable Twin Turbo, Targa option).
So, What are you asking and what do you have available? Most would say the Twin Turbo MkIV Supra is the best pick, but most people don't have $30K USD to throw at a car.