Although Angels do have the ability to transform into any shape YET they do have an original form which is unknown to us. Also there form depends on the type of the angel we are talking about (Angel of torment, Angel of mercy, etc). However I think I'll leave for now because I myself don’t have enough information about these noble creatures (some of the brothers might be able to give you a better understanding on the subject).

This link might help: http://www.islamawareness.net/Angels/murata.html