Blackpool is a more difficult one to judge than Alton Towers. All of Towers' coasters in that poll are modern B&M or Intamins so were easily comparable. However, comparing modern coasters like Infusion against old classics like Big Dipper isn't comparing like for like.
I'd say overall, The Big One gets my vote because it's just so iconic, not just of BPB, but also iconic of Blackpool as a town. It's the modern icon of Blackpool, the 20th century's equivelant to the Blackpool Tower and is great for non riders both inside and outside the park. It's also a fun ride
The Grand National is also a great ride, really intense and total fun. To ride, I'd say it probably beat PMBO, but my vote would still be PMBO for the sheer height, size, speed and appearance of it.
This is probably a really controversial thing to say on an Alton Towers forum, but I actually think that BPB has a better collection of coasters than Towers does, alot more fun anyway. But that's just my opinion.
To summarise, Big One