I wouldn't recommend UA unless you are based in the USA. UA blocks many awards on other star carriers meaning it may be difficult to redeem on flights if you are based outside of the US.

bmi Diamond club currently has some of the best earning and burning rates and they give a 25% bonus on all flights once you hit Silver, whilst LH only award the bonus on certain airlines. Furthermore once you've hit 55k status miles, any status miles above this are converted to destination miles.

So for a typical C class flight that earns 2x miles, you would earn 4x miles + 0.25x bonus! Take a look at the bmi forum for more info.