If you want to rent one, the ones that are rented by First Beats are excellent.
They are the proper medical ones that middies and doctors use. They're very new and I've found them better than the one my GP uses in terms of sensitivity and sound quality.
I used them with both pregnancies and could get the hb from 12 weeks with both. I've got an anterior placenta with this bub and still haven't had any problem hearing him with these dopplers.