I do not mean to make this a poll, but a discussion.
It seems that some aircraft manufacturers have preferred engine manufacturers.
I would say that Boeing's preferred engine manufacturer is GE. They have made exclusive agreements with GE for the 777-200LR, 777-300ER, and 747-8. Originally Boeing wanted to only offer one engine choice for the 787, and GE was considered the most likely candidate.
For McDonnell Douglas, Pratt was definately their preferred engine manufacturer. For the MD-80 and MD-90, they chose P&W and IAE (which Pratt is a part of) over CFM.
Airbus is a tough call IMO.
[Edited 2006-06-15 17:33:30]