One Mile at a Time summarizes some of the best and worse uses of American Express Membership Rewards points. I agree that in general Air Canada and ANA are the best transfer options, though of course it’s best to keep the points in American Express until needed to preserve flexibility (and of course both of those programs have mileage expiration rules).
But something not mentioned in the post is a really neat feature of the program — that via the Membership Rewards website you can link anyone’s frequent flyer mile account you wish and once so linked transfer points into that account. You can have more than one of a particular program linked at the same time. So drop 1000 miles in a friend’s account to top them off towards an award, and then 10,000 into a cousin’s, and finally more miles into your own. I believe this isn’t actually permitted under the rules, but it’s technically permitted by the website.