Once you get to the 30MB/s speed, you should be pretty much set for video on the 1DX. For stills, how often are you going to be shooting very large bursts? If you're going to be doing lots of sports, I'd definitely say get the top end UDMA7 cards (such as the Lexar 1000x), since you'll likely be doing lots of bursts and writing out lots of images to the CF card in a short space of time.
If you're just doing a few frames, pausing, a few more, you can get away with a much slower card. Look at the 60MB/sec cards, I've had a good experience with the 32GB Transcend 400x (60MB/sec write) cards on both my 5d2 & 5d3.
The higher end SanDisk and Lexar seem to be the top brands, but like I said, I've been good with Transcend.
In terms of sizes, if you've got the money, I wouldn't go for cards less than 32GB, and go for 64GB if you don't want to carry around as many cards. Just remember, download them to your computer often, and back them up often. Just because it's from a good, name brand doesn't mean they don't fail ever. It happens, every manufacturer will have a few bad cards here and there. Just from a good one, you likely won't.
