Hi @hongweiyin,
Pre-trained classifiers are available for both SILVA and Greengenes on the QIIME2 resources page.
In the tutorials we generally provide examples with the Greengenes classifiers merely because they are smaller and thus faster for running quick examples.
In practice, QIIME users and developers have many different priorities and opinions on which reference database is right for them — you can see some discussion here and here.
Very glad to hear that you are enjoying QIIME2! Enjoy the journey! 
