There is no right answer, just a lot of options all as valid as each other.
Removing weight starts off as free, but can evolve into a lot of work and expense and is not always the best way to go on your daily driver as it can make it noisy and uncomfortable.
Where are you starting from? If you have a small, light car then sticking to 4 or 6 cylinders is probably the easiest way. With a big, heavy car you need a lot of torque which usually means a V8. This, however, is not definitive but there are very few fast big cars with small displacment powerplants.
A lot of Jap cars are running in the 10s, 9s and even 8s using in line motors, albeit with one or two turbos, superchargers and/or nitrous.
My friend's Ford Anglia (Ford Pop shape) is over a tonne, but has clocked 10.6@131mph after only three runs. He has a 351 V8 Cleveland in it pushing out around 550bhp. Once he gets his 60ft time down he will be close to the 9s. Then, of course, he can switch on his nitrous system
You could, of course, fit a big block Chevy into a Fiat 126, but that has already been done.
Do you want to follow tried and tested methods or plough your own furrow? By following other people you may get close to going as fast as them but not faster.
Are you looking to go faster or FAST? There is a very big difference in work required and the cost involved in going from lower to upper case