Really depends what you're engaging them for and the size and scale of your organisation. If it's for a very sepcific, discreet piece of work, then a niche player is potentially best placed. If it's part of a wider, integrated change program then the Big 4 may be better.
Also depends on your organisation. The Big 4 tend to want to work with bigger, prestigious companies. If you don't fit into that category you may not get their best people. Niche players will, potentially pay more attention to a smaller client.
In terms of the Big 4, in my humble opintion, I'd go: Deloitte, PwC (but expect them to grow via Mr Unwin as that is his background), KPMG, E&Y.