There is no stand out choice when it comes to J2ME GUI frameworks. It really depends on the requirements of your project and what fits best.
I have used a commercial framework from Paxmodept, which worked well and included the source code. In addition to J2ME Polish and LWUIT there are a couple notable open source alternatives:
- J4ME looks decent and importantly has active development.
- Fire allows you to specify the look and feel using xHTML & CSS and also has active development.
For the open source frameworks, download the source code and create simple hello world apps. It shouldn't take long and will give you some insight into the pros and cons of each framework.
If you do use a commercial framework it is important that you have access to the source code as you'll most likely need to make small changes to fit the exact needs of your project.