488 I think is probably one of the quickest cars for S1 (starts out right at the top of S1). Really fun to drive at any speed although it is quite tail happy, particular in 1st and 2nd gear. Tops out at around 210 MPH stock and should be able to beat most S1 cars in a straight line while still being pretty good in the corners.
The Viper ACR, meanwhile, I think is an amazing car for high speed circuits. It is miles ahead of most of the hypercars in the corners due to it's immense downforce although it lacks the power (645HP stock, but can be tuned to around 1200HP) and top speed of most cars in S2. You could probably take around half the corners on Goliath without using the brakes, and quite a few could even be taken flat out, which gives it a big edge over standard road cars. So much so that I was able to use a stock version to catch up to and overtake an S2 tuned Countach HE online, and make enough of a gap to make him quit the race. It's also quite good in slow speed corners as well, as the huge rear wing helps to prevent the car spinning out easily.
Note: The Viper ACR I'm referring to is the 2016 model that you get in one of the DLC's. I haven't bought the 2008 model yet.