It's hard to really recommend one because it all depends on how you want to use them. I'd suggest to download a couple that look good and seem to do what you want them to do (and not too much more - there are quite a few themes that claim they're particularly user-friendly and flexible because they have all the functionalities pre-installed. They tend to be all cluttered up in the backend, so I'd advise against them), set up some posts and pages and see how well they work for you.
Things to watch out for:
- Make sure it's really free, not just freemium or something. Often, the free versions of pay themes are good enough if you know the php and css to make up for the functions they're lacking (I've had some great experience with several themes by Catch), but it can be a hassle. (If you aren't sure, open the Customizer. If it tells you about all the great custom options you'd have if you upgraded, it's freemium.)
- While you've got the customizer open, see what you can do with it. Can you upload a header image? Choose a layout? Set all the colors? The fonts? Not all themes allow for that. (Again, not a problem if you know css, but you might not want to engage with that.) Some themes also have their own custom options in the 'Design' or 'Tools' section of your backend.
- While browsing for themes, take note of when they've last been updated. This will tell you if they're still being maintained and supported. Oh, also look at the support forum for how current the posts are and if questions get answered.
- Generally, the simpler a theme, the better for maintenance. It also makes a differecnce for your performance. Think in advance about what you want your site to do. The closer you get to exactly that, the better. It can even be less. There are plugins to make up for missing functions but getting rid of clutter is harder.
- By now, most themes should be responsive so it's not a pressing issue anymore, but check for it anyway. There are links at the bottom of the Customizer for a simulated tablet, desktop and phone view.
Bonus points if you really want to get down'n'dirty with it:
- If you're php-savvy and are planning on customizing your theme, check out the file structure and the functions.php and style.css for signs of Making Things Too Complicated.
- Also, if you customize it, use a child theme. You can either look if there's one already being offered by the programmers or make one yourself (there are instructions in the WP documentation).There's also a plugin that takes your customizations from the parent theme and makes a child theme.