I have thought about this before and its hard to argue against the 1990s being the best decade for cars.
In the 1990s you had all the technology from the 1980s but it had evolved and was much more advanced. It was the first decade when most cars started to be built with computers which allowed for better design therefore better cars. It was also the pinnacle of car technology before the driving assists and regulations we see today took over.
The cars were advanced but were still raw. Look at a Porsche 993 and compare it to a new one; the 993 has much smaller A-pillars, the 993 has basically no driving assists, the interior is nice but isn't meant to look great, and the driver feels more connected with the car. Then compare the 993 with the 964: The 993 has more advanced suspension which means better handling, the intake manifold on the 993 is better designed to give it more power, the engine uses lighter components, an improved exhaust system allows exhaust to flow more freely.
Do this with nearly any car and you will come to the same conclusion. compare a modern Civic to an EG or EK Civic, compare the 7th generation Celica to the 6th generation Celica.
Sure a new 911 will be faster and a new S-Class will nicer but that does not mean a new 911 will be more enjoyable than a 993 and that does not mean a new S-Class will be as iconic as a W140.
The 1990s were the last time car companies actually cared about truly making cars enjoyable to drive. The technology was more advanced than the 1980s and prior which meant the cars were better than anything that came before them but they still had that connection with the driver that every sports car company is trying to achieve today.
And as a side note the 1990s were really the last decade in which you had the truly great super cars. Not only was the McLaren F1 released in the 90s but you also had the CLK GTR, 911 GT1, Ferrari F50, the Lotus GT1, and others. Sure today there are some similar cars but they aren't race bred like these cars were and there hasn't been anything like a CLK GTR, McLaren F1, or Porsche GT1 released since then.