What car is the fastest (when upgraded) in GTA V and where can I find it?
However I prefer the Overflod Entity XF as it has significantly better acceleration and traction - which matters more than top speed IMO.
Both cars will occasionally spawn in the game world (generally around the richer areas of Los Santos) but if you do steal one you cannot "own" it. The Adder spawns frequently in front of Sessanta Nove at Portola Drive, Rockford Hills.
To own one of these cars you need to buy them from Legendarymotorsport.net - and they're expensive with the Adder at $1,000,000 and the Entity XF at $795,000
It's reported that you can own the Entity XF via the mission "I Fought the Law" but this seems like an exploit of sorts.
And if you want a fast 4 seater car check out my question about that here: What is the fastest 4 seater car in Grand Theft Auto 5?