What's the fastest and easiest way to Disneyland from Royal Plaza hotel in Mongkok?

MTR is the best way to go. Walk from the hotel to Mongkok East Station and take East Rail Line to Hung Hom. Then take West Rail Line to Nam Cheong, where you can take Tung Chung Line to Sunny Bay. Change to Disneyland Line to the resort directly. All transfers are quite easy and you can finish in stations. The whole way takes around 45min and 24HKD.