I have stayed at the Royal Plaza in Mong Kok several times over the last year after trying other hotels in Hong Kong. Each stay has been consistently excellent. There are many good reviews about this hotel so instead let me share some insight, especially for US travelers.
First of all, Hong Kong is very easy for English speaking people to get around. The mass transportation is well marked and all stops are announced in Cantonese, Mandarin, and then English. Do not feel like you need to stay near tourist trap alley. You will be glad that you did not. I found the hotels in Tsim Sha Tsui to be noisy, dirty, and always hawkers standing around outside trying to sell fake watches. The Royal Plaza is a welcome retreat and only a 15 minute subway (MTR) ride away.
The Royal Plaza is a short walk from the subway (MTR) and train (KCR) stations. Those traveling into ShenZhen will enjoy the recently opened border crossing at the KCR Lok Ma Chau station. The hotel is attached to the Grand Century Place mall which houses many restaurants including the Spaghetti House (in case you need a break from Chinese food). There is an ATM machine in the mall right near the KCR station (sort of under the escalator).
One of the rituals I enjoy while staying at the Royal Plaza is an early morning walk past the fragrant flower shops and the chirping of birds in the bird garden. Puts me in a good mood. Both are right across the street from the Royal Plaza (take the elevated cover walkway).
I have stayed in several different rooms. The 19th floor is the executive floor which has the same standard room as elsewhere in the hotel but also has a private lounge where you can enjoy cocktails, appetizers and breakfast (they charge for breakfast). If you can upgrade free to a suite you will be pleased. There are different sized suites, with the ones at the higher floors being the largest. Even the standard room is a good room.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.