French - Very difficult. Requires you to know a lot of the grammar to a high standard and really requires a passion for the language to do well.
Fine Art - Very time consuming. If you do this it will take time from your other subjects unless you are really well organised.
Psychology - Requires you to learn a lot of case studies, so you need a good memory and good essay writing-skills.
In the end, they're all going to be difficult, but take whichever you feel you'll enjoy the most, your AS subject doesn't really matter for medicine.