The answer depends on your goals. Do you want to generate stories online? In broadcast (radio/TV) media? Print media? Industry publications? Or maybe you want to position yourself and/or your company as a leader in your field? Or drive traffic to your website or create an SEO boost for your site? Or meet financial disclosure regulations?
I’m going to assume your goal is to generate news stories. The best way to earn media attention is through very specific targeting of your press release to individual journalists who cover your beat (the subject area your press release is about, such as business, or womens’ issues or motorsports, for example) AND the topic of your specific release. I don’t know of any press release distribution services can promise that (even the paid ones). Their targeting is not very narrow, and most don’t have the ability to target recent related stories.
Most public relations pros like me subscribe to a media database service that allows us to do this granular level of targeting. Unfortunately, if you’re interested in free distribution, the fees for these databases are probably out of your range. However, you can identify journalists manually by making Google News your friend and searching for recent stories about your topic. Most online news outlets post journalists’ email address or Twitter handle, or offer an online form for contacting them.