I've stayed almost exclusively at the Langham Place over the last couple of years (currently have a further 5 stays booked before the end of the year). I really like the club rooms which as someone mentioned previously are a little tight but fine for a few days.

Only downside if you're taking your mum is that Mongkok is a very busy place. If she doesn't mind lots of people and loves markets and mixing with HKers she'll have a great time, if she's not used to crowds then maybe somewhere like the Mandarin Oriental (another gorgeous hotel) might suit her better.