For most of us, our own two feet are by far the best transport in the narrow alleys and pedestrianised squares of tiny Valletta. If you are not very mobile or have luggage, though, there are City Cabs (+356 21 33 3321) - electric golf-buggy-style white mini-taxis with a fixed price of 2 euros per person wherever you are going within the walls of Valletta. Alternatively there is the tourist option - the horse-drawn cab or karrozin. Set a price before boarding. The minimum is about 7 euros.
Valletta is in Malta. For general advice on getting around Malta, see my How to get around Malta page.