Disneyland hotel for us. We love the water slides and pool area, the magical headboards, the restaurants, the theme, the characters they have out and about, the proximity to the monorail, and just the overall feel of it. There’s nothing like watching the fireworks with the headboard nightlight on, snuggled together in cozy pajamas with their new mickey/Minnie plushes after a day of playing and swimming. It truly is our happiest place on earth. Our kids are 6,4 and 1 and I hope we have many more years of magical Disneyland hotel trips to come!
If the kids are older (old enough to go into the parks on their own, so 14 and up), we like the GCH. That private entrance right into DCA by GRR is such a luxury -- especially if you have a kid who's really into GRR. The kids can ride GRR repeatedly (!) to their heart's content, get absolutely drenched, then just come back to the room to dry off, get into clean clothes, and hit the parks again. If that same kid is a big fan of the monorail, a courtyard view room where the monorail goes by regularly is a nice perk. We haven't stayed at the DLH since Tangaroa Terrace opened, but our family really enjoyed WWS and Hearthstone Lounge. And it was so nice to enjoy a luxurious dinner at NR, then just waltz up to the room.
I have stayed at all three. I prefer GCH for the proximity to the parks, but it just barely edges out DLH. DLH has rooms that are more charming for young children, with the light-up headboards. The hotel itself also has a fun sense of whimsy. GCH has great pools along with being directly IN California Adventure for easy access. Both of these hotels have great character meals.
For a kids perspective, DD6 loves them both. She also loves Paradise Pier, but it's the lowest on my list due to distance. (Of course, the walk is still completely manageable, but it suffers in comparison.) The reasons that PP ranks so high on DD's list are the beach ball/Pixar ball pillows, the fun elevator system, and the CMs there who made her stay special.
GCH has the highest value for me in that it's the closest hotel to the parks (with the private entrance too). This is HUGE when you have a cranky kid needing a nap, or when it's hot out and you don't want to walk so far just to get to your hotel.
I like DLH for the pools and the fact that the rooms are larger than GCH (it's an older hotel) along with the flourishes @DisneyPiPhi lists above.... but the hike is real, and sometimes the monorail stops for whatever reason (maybe I have bad luck) so it's not as convenient as one thinks.
Proximity is huge for us, so we either stay under our VGC contract, or when we're out of points, we just book at Tropicana Inn & Suites on Harbor, which is one of the closer hotels (recently renovated). We probably wont' stay at DLH again until DD4 becomes DD10, hah.
We've never stayed at the Paradise Pier but we have stayed at DH and GC. We prefer the Disneyland hotel due to the bigger rooms and they are quite a bit brighter. The grand is nice but not as nice as the Disneyland hotel imho. The Disneyland hotel just seems special. And to top it off the Disneyland hotel is more affordable. But the grand is very nice and is closer to the parks unless your room is way on the opposite side of the hotel. If you get a room there, the walk is pretty long. But most people will likely enjoy both of them so you can't go wrong either way.
GCH is the choice for my family. Riding Grizzly Rapids over and over at the end of the evening, then heading back to the room (the entrance to the hotel is steps away from Grizzly Rapids) while non riders head to DTD to grab food from Napolini or Tortilla Joe's Taquaria.