The double d simply has 2 fans compared to the single one on the other card. And that's the card i'd recommend going for out of the 3.
In terms of the best 7870 definetly has to be this:
MSI R7870 HAWK Radeon HD 7870 GHz Edition Video Card - Newegg.com
Has an overclock at 1100mhz (most 7870's are clocked between 800-1000 most commonly being in the 800-900 point though theres a few with 1 1050 clock) and has great cooling.