Ideal Teaching Mix as a Postdoc? I'm a math postdoc at a research university, and I have 1-1 teaching for the next two years. What would be the ideal mix of courses to (1) prepare me for a higher teaching load in the future (hopefully as TT faculty), (2) demonstrate my teaching ability to hiring committees (if it matters), and (3) still allow me to concentrate on research?

I'm considering requesting Abstract Algebra II and Linear Algebra next year. My only teaching experience so far was one semester of Linear Algebra in graduate school. It went well and I enjoyed it, but it was a while ago so I think I'll have to spend some time getting used to teaching again.
First it depends on the type of tenure-track job you're seeking.  For some jobs (e.g. liberal arts college or regional university focused more on teaching than research) your teaching will be considered more seriously than your research.  You should also know that for many of these jobs, having a postdoc at a research university is "one strike" against you: they will still consider you, but they really want to see and believe that you are interested in their institution because of a true interest in teaching, not just as a backup if a research job falls through.  

Second, teaching just one course as a graduate student is a bit behind the curve for math students at American universities.  With this kind of background you would have to do something rather heroic to be seriously considered by teaching-oriented programs -- not necessarily teaching more courses, but perhaps exploring a combination of teaching and outreach or taking on more leadership responsibilities within the teaching life of the department.

If you are looking for a research-oriented job then my feeling is if you have good rather than bad teaching evaluations, that will serve you reasonably well.  The academic job market is certainly very competitive, but for a research job your time and energy is probably better spent concentrating on your...research.  

I don't think a higher teaching load needs to be prepared for.  But it's a good idea to get experience teaching calculus of various sorts, since most math faculty I know teach a lot of that.  

As a postdoc at a research university with a 1-1 teaching load, I would encourage you to take some teaching assignments that are routine in the sense that you will not spend too much time engaging with or preparing the material (again, calculus is good for that).  For your own interest as much as your future career, it might be pleasant to alternate those with some more interesting teaching assignments.  If you can teach one graduate course while you're there, that will probably be a fun, rewarding experience, and it will strengthen your connection with the graduate students there (many of whom will end up being your future colleagues). Teaching an advanced undergraduate class is also a good data point for understanding what undergraduate math majors are really like.

In summary, there are no clear answers here, and for a future research job your choices will probably be informed more by your own predilections and a desire to broaden your perspective rather than to specifically increase your chances of being hired at a research university.