My top 3 in order have to be:
1. QF's FASA/JASA (never booked a MASA). Brilliant use of points and kept me as Platinum for a couple of years. Earned and burned a lot when closure of this loophole was announced.
2. AAdvantage - initially the Plat Challenge and then (when it was still available) transferring CC points to my AAccount where it was included towards Lifetime Status. Once I have LTG with QF done, then I'll start crediting back to AA to get Lifetime Gold. That may take a while but my retirement flying won't be at the back of the plane.
3. DSC's. With the loss of number 1 above, there needed to be something to fill the void. Whilst the majority of my flying is now VA, when it's DSC time and eligible, I participate

. This is helping get me faster to LTG.
Honorable mentions:
1. VA Status Match. I was a dyed-in-the-wool Qantas fan until Brett Godfrey cajoled me and I finally succumbed in August 2011 in the second round of status matching and haven't looked back.
2. VA Family Pooling. Well none of my kids or wife needs the points or status credits, but they benefit from me having Gold or Platinum status when we fly together. They allow me my predilection as it benefits us all.
3. T-80 seat selection. Particularly now as a Lifetime Silver member, normally it'll seat me in ~ Row 16 in Economy when I book, so I assiduously calculate T-80 and put in a reminder. With my recent DSC-inspired slew of QF flights, for those where a points upgrade hasn't cleared, this is a must. Case in point was yesterday, checking a booking I have (Saturday BNE/SYD). Instead of 16C I had been pre-assigned, I was able to select 4C at T-80.