multivariable calculus (calc III) - take those calculus I and II techniques and apply them to a sitution with several variables, which is much much more common in practice, also gain intuition about 3 and higher dimensional problems
probablity and stat with calculus (pref. multivariable) - if you took either of these courses without calculus you have only seen half the potential application, clear application to analysis of research data and clinical trials
linear algebra - like novawildcat mentioned this has application to so many fields, will complement strongly your ability to deal with multivariable situations by introducing ways to solve big systems of equations, also introduces important notion of eigenvalue/vectors that is useful in pchem (and diffeq more generally)
diffeq - this was really an eye opening class for me, there are so many relations that are best described using a first or second order differential equation (fluid flow, temperature flow, waves, stock price movement), it gives you a very powerful tool to use in a model context