The Sheraton Skyline is a very nice hotel and the Smart rooms are great if you need to conduct business while there. It is more expensive than the regular Sheraton though.

The Hilton is walking distance to terminal 4 as I recall. It's a good choice if you will be leaving from terminal 4.

I usually take the Hopper bus to the hotel. But as I always like that extra hour of sleep in the morning, I wait til about an 45-50 minutes before the flight, then hire a car from the hotel for about 5-7 GBP to go to the terminal. It's much faster than the Hopper bus which can take over 20 minutes depending on what hotel you are in and what terminal you're going to.