While I wouldn't consider myself an expert on the topic, I do have a lot of experience with the genre and definitely know what it is that I like and dislike. For example, I dislike anything with the name Andy Sidaris or Roger Corman associated with it. This will undoubtedly conflict with other peoples' expectations of the genre, and my observations and conclusions may even piss a lot of people off. But that's why they're my recommendations, and yes, they are biased and extremely subjective.
What makes a piece of girls with guns entertainment work for me is the way in which women are presented and the conviction of those women - it's all about attitude. If the creators present and treat their female characters with admiration and respect, that shows through to the audience. Additionally, those female characters must have enough inner strength and conviction to back up their actions. Without this attitude, the genre tends to slip into exploitation territory.
Having said that, my definition of girls with guns entertainment needs to be redefined so that it encompasses any action genre that features strong female characters. This includes girls with swords, girls with kung fu, and even girls with magic wands. The tools are irrelevant - it's the way the tools are used that counts.
So, without any further delay, here are my top picks. It's by no means an exhaustive list, and even skips entire sub-genres altogether.
And if you want the low-down nitty gritty on all of the girls with guns films I've seen, be sure to check out my movie reviews page.