Anybody with substantially less power than what you have...
I have an Audi S4. So anyone with much less than 340hp is slow.
If I had a 600hp AMG, then anyone with much less than 600 hp would be slow.
However, I disagree with the "more power is better" argument. Instead, I agree with Mikemike: It is more fun to drive a slow car fast than a fast car slow.
Call me crazy, but I just can't comprehend people who want more than like ~250hp. Unless you are going to spend time at the track, you simply cannot use the full throttle in a car with too much power. My S4 is way over-powered and therefore, IMO, not much fun to drive. Thinking of getting a Mazda 5. Bigger. MUCH MUCH slower. And I suspect - I'll enjoy it more.