M4 or FP3Watch
I would generically go with FP3 though,I feel m1-m3 is enough mechanics for now.
Such great topics like vectors, complex numbers, and then group theory shows it's ugly face. The questions are also always like 8 marks. Vectors can be fun.
My favourite though by far is differential equations
A group is a set of elements (e.g. matrices, complex numbers, integers, functions etc.) together with a binary operation (e.g. matrix multiplication, multiplication, function composition, etc.), which satisfies the four axioms of a group: closure, inverse, identity and associativity. There are other important concepts such as the representations of a group, but that would come later.
In the physical sciences they're useful in places such as determining if a molecule has a dipole moment or not (as the symmetry operations of a molecule constitute a group - the point group), if certain normal modes are active in IR spectroscopy, etc. along with a whole host of other things.
Obviously there is mathematical significance besides their use in physics but that's for a mathematician to talk about.