I've done 2/3 and these are all over the place in terms of difficulty.
Driving 250 yards is really achievable, as someone with just about any body type can strength train and learn the technique involved to do it.
Dunking, meanwhile, especially two-handed, is going to be literally impossible for a huge percentage of the population. For a number of years I could one-hand dunk at 6'1", but I still couldn't throw the two-hander down. Now, it's not like I was the greatest athlete in the world, but I ran about a 56 second quarter and a 2:07 800. And I still couldn't two-hand.
And yet, as difficult as a two-hand dunk is to achieve, I would rate a sub 15 5k as even harder. My pr was 16:53 in high school and I think 16:00 (ok, 15:59) was likely my ceiling had I trained longer or stayed in shape over the summer.
Still, even though I believe the sub 15 5k is the most difficult by a significant margin, I'd still pick the ability to dunk two-handed for myself. It's something just about everyone can appreciate and respect. Tell people you ran a sub 15 5k and most people don't know enough to be properly impressed. Plus, it's not easily provable right in the moment.