PWC / Deloitte: Big-4 finance transformation practices. Great name to have on your CV later in your career, plus lots of opportunity to move into other areas of the firm if you want to.
Hackett: More of a specialist play, but very well respected for what it does. More of a focus on benchmarking / best practice than on pure transformation work, but that's my judgement from the outside looking in. Insiders may have a different view.
IBM: Finance isn't their core business, more of a systems focus. Wouldn't be on my short list.