For speed, the above-linked Sandisk 100Mb/s UDMA7 card is the fastest they make. It's 128GB, meaning >4,500 18 MP RAW images on a card. It's expensive - $880 at B&H.
I prefer smaller cards and more of them (the don't put all your eggs in one basket philosophy), but a pair of those pricey cards in the 1D X should be safe.
Personally, I think it's best to balance speed, size, and cost. I have a pair of Sandisk 32GB 90Mb/s Extreme Pro cards I plan to use in my 1D X. I was fortunate enough to get them during the heavy discount late last year - $100 each for the pair, they're $218 each now.
Given the only modest difference between 60 Mb/s and 90 Mb/s in terms of burst size and buffer clearing, I expect the difference between 90 Mb/s and 100 Mb/s to be minimal. Some time back, I
empirically tested various Sandisk card speeds and sizes on the 7D and 5DII. In terms of backward compatibility, as stated above make sure that your 7D/5DII firmware is up to date - that fix supposedly applied to UDMA7 cards, but as you'll see in my tests, the older firmware also throttles the 90 Mb/s UDMA6 cards.
