Thought this would be a good topic for international travel so I split it from the luggage thread.
I am a PPS member with Krisflyer of Singapore Airlines. I throw in whatever Star Alliance flights I take into it. So far, Singapore Airlines has not bugged me for having overweight, even when I fly economy and have 50kg (this was when I was a Solitaire member).
http://www.singaporeair.com/saa/en_UK/cont...itaire_life.jsp
The benefits are pretty much like every other airline's high flyer status benefits, but I find that they are pretty lax with the luggage rules beyond those rules. I've travelled with 5 bags of 70lbs a few times as well as 3 carry on bags including spare housing. One thing they are VERY stingy about is upgrades from economy. I get one every few months but it was better with Cathay Pacific's Marco Polo Diamond.
Cathay Pacific's Marco Polo Diamond level is similar but they are more eager to upgrade you but pretty strict on the luggage, especially at Hong Kong Airport.
https://www.cathaypacific.com/cpa/en_INTL/ffp/mpo/benefits
For Europe, Virgin Atlantic takes care of their customers very well too. I get good treatment as a Krisflyer member. And Virgin definitely has the best lounge at Heathrow, even if Heathrow sucks! 
Note that while airlines do value frequent flyers, at many airports that have general staff working all other airlines, your privileges are not as recognized without asking for the airline staff directly. This was especially so with draconian luggage airports like many australian airports and in particular South Africa's JNB and CPT. Always ask for the airline staff NICELY and things will be taken care of.
Still I wouldn't travel with as much crap since often to get to better destinations, we also have to take domestic airlines and puddle jumpers. So pack lightly.
Conversely, the worse frequent flyer program has to be Delta. They charged me for a bag that was 5 lbs overweight even though I was a Gold member. Last time I flew with them too.