Which form is to be preferred?
- I would like to ask you a favour.
- I would like to ask you for a favour.
According to this Google NGram, all of the following forms are in use:
- ask you a favo(u)r
- ask you for a favo(u)r
As you can see, the forms without the preposition for are more common (with “ask you a favor” generating the highest results). In common use, one may conclude that this is the preferred form.