In my opinion the base game is solid and does not need any expansions. However, as an owner of all the expansions I can tell you that I enjoy the Heat expansion. It offers Gear and Specialists which are fun and add to the theme, giving players more options. None of the expansions are "game changers" but when combined together, they greatly add to the overall experience, and I would recommend them all if you will be more than a casual player.
I am also hoping for a deduction expansion. Depending on how this is implemented, I think it will have the potential to push this already great game to new levels.