Recommend NGK Iridium IX plugs.

The e53 comes standard with either NGK BKR6EQUP or Bosch FGR7DQP plugs. The Bosch Platinum 4's look similar to the FGR7DQP but after initially running these plugs at 42K miles, the car felt sluggish, MPG went down, and i noticed no gain in performance whatsoever. Basically, they felt worse than my oem plugs with 42K on them.

Swapped in NGK Iridiums, the car felt refreshed. Sent my Bosch plugs back to the company, received a refund.

The only other 2 plugs I would recommend would be the BMW Bosch or the BMW NGK plugs from the dealer. Stay away from Bosch +4....Search other BMW forums and you'll see many others can relate to the experience I had with those plugs.