There are a lot of plugins that demonstrate Great Wordpress Plugin Development. Some of them demonstrate great development as in the well commented code, Coding style as per the WP Standards and using the WP API with neat tricks that are neither documented and neither you will see them used in any other plugin.
As a WordPress developer my personal top 3 favorites are:
- Log Deprecated Notices
- Core Control
- Monkeyman Rewrite Analyzer
That said, not only plugins give you the idea of how you can write a great WordPress Plugin but if you follow, read and listen to the advices of the Great WordPress Developers and read their code you will learn many cool things. Almost everyone here on WPSE or on other WP forums are awesome WordPress developer but few of my personal favorites are:
- Andrew Nacin - Blog
- Mark Jaquith - Blog
- Dion Hulse - Blog
- Otto - Blog
- Scribu - Blog
Also if you go through the answers of the most reputed volunteers here you will find a lot of, yes a lot of good stuff, ideas and WordPress coding techniques.