Outside of the resorts, the Park Hyatts are usually pretty upscale and luxurious. Usually have fawning staffs and very fine dining.

Grand Hyatts look like W's. All Ikea-type furniture, but usually very sweet bathrooms and lots of trick lighting and such. Very solid and friendly staffs and usually very good dining.

Hyatt Regencies are your average full-service property. Some of the older properties are a little dated, while the newer ones are pretty nice. Nice staffs and good dining.

So Parks are the luxury brand, Grand are great for business travel, and Regencies are solid vacation hotels.