In terms of cooling performance, they all perform very close,
but for Crossfire, the Twin Frozr would be better for Crossfire, it is designed in a way that allows air flow when the cards are almost touching each other, that's why it has a smooth streamlined edge at the top.
The Asus DCII is HUGE, it's a triple slot card! it takes up three slots but YES it is the coolest, it has the lowest temperatures because it's a triple slot, if you want to Crossfire it, you could with another brand as it wastes 3 slots, if you want a single card that runs the absolute coolest, get the DCII, I personally think it's ugly!
The Vapor X is good, although it's only available in 1GB so it can't be unlocked to a 6970, if you aren't planning on unlocking it to a 6970 and want cooling similar to the
DCII triple slot fat Asus, get the Sapphire Vapor X
I prefer the Twin Frozr