I think Flash is a contender for the 5th spot for sure. But he does have significant holes that keep him from being S-tier (definitely A+ material, and if we broke down into S+ tier he would be S-).
For one, his '50/50' is partially unsafe for some characters (the overhead string wiffs and d12 has terrible range)... (Although I think the overhead can be made safe, he no longer can hit-confirm for the same damage).
He can also have trouble setting this up in the neutral if b22 wiffs, which means he has to play a risky b2-RMS meta game, or an even riskier dash in/Sonic Pound/torpedo thingy - to get in game***
***and this is coming from a zoning perspective, who want to get away from flash.
The character is fast and does so much high burst, corner carrying damage that he can be very overwhelming. His mixups are insane (especially if you do not have a fully invincible wakeup). It still gives me a migraine trying to remember when the Sonic Pounds and ambiguous F3s or f3-cancels are supposed to 'cross-up'. Concentration is so important and it really comes down to punishing Flash really hard when you have the opportunities... And there are more opportunities than people realize (compared to a char like batgirl), but they are easy to miss and will be gone in a ... flash.