According to Derek Snyder, a Product Manager in MED (Microsoft's Mobile and Embedded Divsion), who spent some time going through reports and doing his own research, a list of the Top Windows Mobile Developers would include the following companies (in no particular order):
Spb Software House
MpegTV LLC (PocketTV)
DDH Software, Inc. (HanDBase)
Karrier Communications (IntelliGolf)
The criteria included device side components, being the developers of one of the top 50 best selling items in the Handango yardstick, and having more than 1000 downloads on Handango.