I have the Capital One venture card, which gives you 2% back on all purchases. I love it. You have to spend the rewards on travel (or retroactively apply credits to travel expenses on your statement) which is really easy, and is the same as cash back as far as I'm concerned (I have alot of travel expenses).
There are 2 capital one cards: a non-annual-fee version that gives 1.5% back, and the 2% version I mention above with an annual fee. I've crunched the numbers and seen stated several places (nerdwallet.com, among others) that if you spend above something like $7500 per year, the 2% card is the best deal despite the annual fee. For that matter, is one of the best deals anywhere.
And this last point is a BIG one: No international transaction fees. These can get very pricey with other cards.
Now, all of this only applies if YOU get the rewards somehow, not your company. Someone mentioned it earlier, but your best bet may be an affinity card with the airline you will be travelling the most.