My vote should be obvious: I worked on STIS for 5 years. But it's not just loyalty: Hubble lacks a good UV camera right now, and cannot take decent spectra at all in the optical or UV. ACS is a fantastic machine, but I think (I need to do more research, admittedly) that WF3 will cover a lot of the same
So what do you think? Leave comments on her blog; give her some love.