On reflection I'd agree with rkkwan if you have the time and inclination to plan a bit. Public transport is easy. If you're going to do that then get Octopus cards for everyone (you can use them on the subway, bus, trams, Peak Tram (thereby jumping the queue to buy tickets), Star Ferry and Airport Express).

But if you really don't want to do that then a custom tour in limo would be much better than a taxi.

A bus tour is really not a good idea.