Amoung all the Popular standing order items I don't think that there is a best. It actually depends on you and how you want to forward your business.
The IAHBE is a business hub that has numerous ideas for all affiliates who want information on how to forward their business.
The S-Builder is a easy and cheap way to build your downline. SO if you a looking at having a big team then this would be your best option.
The TCredits are good for bidding on actions, buying different items from TripleClick which give you VersaPoints and helps you to have more action points therefore helping you reach the next level.
SO all the popular standing orders are the best it just depends on how you want to forward your business and what you want to do.