How old are your parents?
I've stayed at all 3 properties. The GCH is the closest to the parks and is definitely the most upscale feel of the three, at least in the common areas. However, don't let that fool you into thinking that means it is a short walk. The hotel itself is huge and it can be a long walk just from the hotel entrance to your room. It has a very cool art deco vibe and the pool is the most "adult" feeling of the three. Cleanliness shouldn't be an issue; it's the newest of the three and is very well maintained. It has a private entrance directly into DCA that can be used by any Disney hotel guests, but you won't want to use that entrance unless you're not trying to be there early because the line there is horrendous in the AM. Personally if I were taking parents and could afford it, I'd stay at the GCH unless they are really into Disney theming. Just sitting in the lobby is an experience. It's a beautiful hotel and is my favorite of the three.
The DLH is the most "Disney" feeling of the three. The rooms have more "Disney" touches like the light-up headboards (which are tasteful in spite of how that sounds) and the pool, which has the monorail slides (and is very popular with kids). It can also be a long walk to the room, depending on which tower you're in and where you stay in that tower. The walk to the park is longer than the GCH but it is through DTD, which is nice (as opposed to the PPH which doesn't really feel on property as far as I'm concerned.) It is convenient to the monorail, but don't plan on taking the monorail if you want to be at the park early because it doesn't start running until the park is open. (Some of the GCH rooms have monorail views but the monorail doesn't stop at the GCH. It stops in DTD not too far from the DLH and in DCA only.) It also has Trader Sam's, which is a very cool "Trader Vic's" type bar that DH and I love. Its rooms are larger than the GCH rooms. The DLH is a fun, very "Disney" hotel and is my teens' favorite of the three, and my second favorite.
The PP is my least favorite (my teens didn't like it, either). It's the only Disney hotel that wasn't built as a Disney hotel and to me that was really obvious. To me, it just looked like a regular hotel (and not even a really nice hotel) with some Disney splashed around. The lobby is small and does not make anywhere near the impression that either of the other two make on check-in. However, since it is the least expensive of the three, if you're into views you can get a REALLY nice theme park view here for MUCH less than you can at either of the other two. And the non-view rooms are the cheapest way to stay in the bubble. Some people like it because it has a more laid-back feel than the other two hotels. The pool is ok, but not as nice as either of the other hotels, although it is definitely quieter than the DLH pool. It's the only one of three hotel pools where they don't offer cabana rentals. It's across the street (a wide, busy street) from the GCH so isn't as close. Actually I think by the time you walk from the room, down to the entrance, across to the GCH and out of the hotel, the walk can be just as long as some of the DLH rooms.
Good luck choosing; I'm sure you'll have a great time no matter which you end up at!