It's getting much tougher to amass points with credit cards (and even actual flying). At this point, what i am finding best for me is Citi Hilton which I churn. I'm collecting points to use at the Conrad Rangali in the Maldives. It's the best value I've seen (short of the Hilton somewhere in Oman which I can't remember: a $200 a night room for 5000 points).

Other than that, I use Chase Sapphire Preferred when I get 2 points per dollar. I use Hilton Amex for groceries, drug stores and gasoline (can't remember how many points, but it works out to be more than 2 points per dollar at my very particular Hilton valuation) and Capital One that gives 2 points per dollar. Note, that to get 2 cents in value per dollar you must redeem for already completed travel.

Booking travel through the Chase portal will sometimes give you very good value for your points. Plus Chase is transferable to lots of different airlines and to Marriott and Hyatt. I think the transfer to Hyatt gives much more value than transfer to Marriott. You just can't churn Chase easily if at all.

I really only know about United in terms of travel. I think the current sign up is about 50k miles. That won't get you far. Since UA is changing the way they calculate the miles you get from flying, it will be very hard to accumulate enough miles for trips.

My advice is to choose a goal and then research whether the various point schemes will help you. The cards I've mentioned have no foreign exchange fees.