If "hardest" means tightest racing, it's hard to argue against PS. You have whole fields separated by less than the difference between the top few in the fuel classes.
If "hardest" means trickiest, then I think it's PS as well. Asked Frank Hawley when I was in his class, "is there any class you wouldn't drive?" He quickly answered PS. With zero downforce, it's like skating. Way too easy to end up on the roof.
If "hardest" means toughest to win, I'd say Super Comp. Close racing with MOVs in the 10,000ths, RT expectations beyond sanity, amazing drivers who can race you at the stripe at 175+, and people who've been doing it for decades. I know you all will pooh-pooh it, but it's why Shawn Langdon, Jack Beckman and others are such good racers, years in SC.
If "hardest" means most difficult to watch, I'd say PSB. Just can't get my head around that one.
My $0.02,
Chris