<p>Whoops, should have specified, the GRE part was more directed towards alimpossible.</p>
<p>I wouldn’t ask for a recommendation from a professor who doesn’t recommend me, even if you will be grading for him this coming term (odds are you still won’t have many interactions).</p>
<p>The other two seem like good choices, though it’s difficult to say if the former post-doc or the MechE professor would be better. If the post-doc knew you well and was familiar with your work throughout the term he might be the better choice since he can comment better on your work ethic, ability to work with others, and those sorts of less academic traits. The admissions committee will see you got an A in both of the classes taught by the other guy, and there’s no guarantee he’ll write about how crazy hard his classes are.</p>