My favourite out of Mount St, Motcomb St and Harvey Nichols is definitely Motcomb St. All three have lovely staff and a good selection, I just seem to like the Motcomb St boutique the most.
Bear in mind that they may not have what you want in your size so be prepared to go to the others... if there is something you like and want then ask them to check if the others have it in stock... not sure if Motcomb St and Mount St can check stock on HN but they can each other. They're all within 15-30 minute walk of each other so just wear comfortable shoes!
Enjoy and let us know how it goes!