Where's the love for Kennywood? C'mon people ...
My wife and I (and now our 8-year-old DD) are coaster junkies, and have been all over North America and Europe touring parks. I can say without a doubt Pennsylvania has the best collection of classic wooden coasters (and other rides, for that matter) in the world.
So it depends what you are into. We've been to all of the parks mentioned above, and have enjoyed each one (plus a couple not noted, including Lakemont, where you can ride Leap-the-Dips, the oldest operating coaster in the world).
Knoebels is beyond awesome. It is such a special place - and not just because it has my favourite wooden coaster in the world, the Phoenix. Kennywood is packed with rides you won't find anywhere else, and has several classic coasters, including the fantastic Jack Rabbit, and the highly-thought-of Thunderbolt.
Waldameer, to me, is worth a stop if you're near Erie, because although it's not the biggest park around, it does have the Ravine Flyer II - an insane modern wooden coaster that is consistently voted as one of the world's 10 best.
So you can see where my interests lay, which may not be everyone's taste. Hershey and Dorney are much more the proto-typical modern parks, with cutting-edge rides and fun water areas. We enjoyed them both a great deal.
It seems most people are recommending Hershey and Knoebels, which might make the most sense, as between the two you have a great mix of old and new, and they are just over an hour's drive away from each other.
IMHO, you can't go wrong with a visit to just about any park in that state ...