American Express is having a special right now on transfers to British Airways Avios miles. Until January 31, you will get a 40% bonus for all miles transfer. For example if you transfer 50,000 Membership Rewards points you'll get 70,000 Avios points.
Avios points are especially worthwhile to me because I have the ability to get flight segments for less than the standard 12,500 miles. For example a one way between LAX and Las Vegas would be 12,500 miles on American Airlines, Delta Airlines, and United Airlines. Using Avios points, you only need 4500 miles.
Another really nice redemption for Avios miles is lie-flat Business class seats on the transcontinental flights on American airlines, which is a partner. These only cost 25,000 Avios miles. Using this promotion, this is the equivalent of 17,800 membership rewards miles.
After American Express lost United Mileageplus as a partner, I found difficulty using membership rewards points. This is a promotion I got in on, and transferred 50,000 points.