Sheraton recently opened a new hotel in Krabi. I am considering the Sheraton Krabi, Rayavadee and Pimalai Bay. I would appreciate any advice. Thanks

You have big price differences here. The Pimalia is Koh Lanta which is different area to krabi.
The Rayavadee has a great reputation, the Sheraton is new, some reports said the beach wasn't that great.
I guess if you want to spend the big bucks then the Rayavadee provies top level accommodation and service. However, you are paying for that.
If you intend spending most of your time in the hotel/resort itself and not venturing out much then fine, but if you plan day trips to Phi Phi, Phang Nga and other tours then you may want to look at some very good quality resorts that are far far lower pricewise.
The Sri Lanta on Koh Lanta is great value, the Pimalai is also good.
My own choice for a very good resort with good location/accommodation would be the Tubkaak Resort near Ao Nang Beach, Krabi. It is very Thai style, gfreat service and atmosphere, it will give you all the service standards of the more expensive places.
You can try this local Thai site, they have customer reviews too

http://www.freewebs.com/koh_samui_hotels/krabi.htm

They also have Koh Lanta too. Friends always stay at the Sri Lanta or Lanta island and love it. 'My' favorite in krabi area though is the 'Tubkaak'.
Hope that helps.

As the above message implies, I guess it really depends on what you are planning to do and how much you want to spend. We also considered the Krabi Sheraton for a few nites, but in the end figured we didn't want to spend our time packing and unpacking so just lengthened our stay at Rayavadee-which our kids loved. It is pricey, however, the views from the two Railay beaches at sunset are, to quote the commercial, priceless. While we tend to disbelieve the photos of resorts on the web as unreliably old or deceptive, Rayavadee was exactly as it appears on its site. The only downside may be the food which we found tended to be either bland or overdone.
By the way, Pimilai Resort was heavily promoting itself with pretty substantial discounts at the Emporium Shopping Center in Bangkok. I don't know if this is still the case, but if you are interested, perhaps a better deal can be arranged once you arrive in BKK.

As Dywong says, you can get good local offers and overall you get a far wider choice of accommodations at all levels on local Thai websites and you get access to all thoese resorts that are not listed on the 'mega' travel sites outside of Asia.
I always go for local Thai sites to find Thai places and do the same traveling elsewhere.

I stayed at all three last April. Sheraton is Ok but rooms dont compare to the other 2 and are set back quite a long way from the beach. Rayavadee is very nice, but in my view overpriced and water quality doubtful. The Pimalai is the only hotel in the world that I've returned too. Its great, but obviously a bit quiter. If budget allows, book a suite on the beach.