Here you go:
The best open source office software: LibreOffice
LibreOffice hasn't changed its user interface very substantially for a very long time. In the recent LibreOffice 5.3 release, they introduced a new interface option, which they call the MUFFIN (My User Friendly Flexible INterface).
The new interface has several modes, including Single Toolbar, Sidebar, and Notebookbar. The last mode, Notebookbar, is interesting. This is very similar in concept to the Microsoft Office Ribbon. People who come from an Office background and are used to how Ribbon behaves, and how it changes based on what you are working on, should like the Notebookbar setting.
To comment on the new interface: I think this is an interesting and welcome direction for LibreOffice. I don't think the current user interface is bad, but I think the proposed changes are a positive step forward. The new MUFFIN interface is flexible and supports users they way they want to use LibreOffice. I think it will appeal to current and new users, and "lower the bar" for users to come to LibreOffice from Microsoft Office.
The best open source photo editor: GIMP
I use GIMP at home for a lot of projects. Most often, I use GIMP to create and edit images for my websites, including the FreeDOS Project website. Although we've recently turned to SVG where possible on the FreeDOS website, for years all our graphics were made in the GIMP.
A few years ago, I asked readers to suggest programs that have good usability (I also solicited feedback through colleagues via their blogs and their readers). Many people talked about GIMP, the open source graphics program (very similar to Photoshop). There were some strong statements on either side: About half said it had good usability, and about half said it had bad usability.
In following up, it seemed that two types of users thought GIMP had poor usability: Those who used Photoshop a lot, such as professional graphics editors or photographers. Those who never used Photoshop, and only tried GIMP because they needed a graphics program.
So GIMP is an interesting case. It's an example of mimicking another program perhaps too well, but (necessarily) not perfectly. GIMP has good usability if you have used Photoshop occasionally, but not if you are an expert in Photoshop, and not if you are a complete Photoshop novice.
The best open source media player: VLC
I haven't used VLC, part of the VideoLAN project, in a few years. I just don't watch movies on my computer. But looking at the screenshots I see today, I can see VLC has made major strides in ease of use.
The menus seem obvious, and the buttons are plain and simple. There isn't much decoration to the application (it doesn't need it) yet it seems polished. Good job!
The best open source video editor: Shotcut
This is a new project for me. I have recorded a few YouTube videos for my private channel, but they're all very simple: just me doing a demo of something (usually related to FreeDOS, such as how to install FreeDOS.) Because my videos aren't very complicated, I just use the YouTube editor to "trim" the start and end of my videos.
Shotcut seems quite complicated to me, at first glance. Even TechRadar seems to agree, commenting "It might look a little stark at first, but add some of the optional toolbars and you'll soon have its most powerful and useful features your your fingertips."
I'm probably not the right audience for Shotcut. Video is just not my interest area. And it's okay for a project to target a particular audience, if they are well suited to that audience.
The best open source audio editor: Audacity
I used Audacity many years ago, probably when it was still a young project. But even then, I remember Audacity as being fairly straightforward to use. For someone (like me) who occasionally wanted to edit a sound file, Audacity was easy to learn on my own. And the next time I used Audacity, perhaps weeks later, I quickly remembered the path to the features I needed.
Those two features (Learnability and Memorability) are two important features of good usability. We learn about this topic when I teach my online class about usability. The five key characteristics of Usability are: Learnability, Efficiency, Memorability, Error Rates, and Satisfaction. Although that last one is getting close to "User eXperience" ("UX") which is not the same as Usability.
The best open source web browser: Firefox
Firefox is an old web browser, but still feels fresh. I use Firefox on an almost daily basis (when I don't use Firefox, I'm usually in Google Chrome.)
I did usability testing on Firefox a few years ago, and found it does well in several areas:
Familiarity: Firefox tries to blend well with other applications on the same operating system. If you're using Linux, Firefox looks like other Linux applications. When you're on a Mac, Firefox looks like a Mac application. This is important, because UI lessons that you learn in one application will carry over to Firefox on the same platform.
Consistency: Features within Firefox are accessed in a similar way and perform in a similar way, so you aren't left feeling like the program is a mash of different coders.
Obviousness: When an action produces an obvious result, or clearly indicated success, users feel comfortable because they understand what the program is doing. They can see the result of their actions.
The best open source email client: Thunderbird
Maybe I shouldn't say this, but I haven't used a desktop email client in several years. I now use Gmail exclusively.
However, the last desktop email client I used was definitely Thunderbird. And I remember it being very nice. Sometimes I explored other desktop email programs like GNOME Evolution or Balsa, but I always came back to Thunderbird.
Like Firefox, Thunderbird integrated well into whatever system you use. Its features are self-consistent, and actions produce obvious results. This shouldn't be surprising, though. Thunderbird is originally a Mozilla project.
The best open source password manager: KeePass
Passwords are the keys to our digital lives. So much of what we do today is done online, via a multitude of accounts. With all those accounts, it can be tempting to re-use passwords across websites, but that's really bad for security; if a hacker gets your password on one website, they can use it to get your private information from other websites. To practice good security, you should use a different password on every website you use. And for that, you need a way to store and manage those passwords.
KeePass is an outstanding password manager. There are several password managers to choose from, but KeePass has been around a long time and is really solid. With KeePass, it's really easy to create new entries in the database, group similar entries together (email, shopping, social, etc.) and assign icons to them. And a key feature is generating random passwords. KeePass lets you create passwords of different lengths and complexity, and provides a helpful visual color guide (red, yellow, green) to suggest how "secure" your password is likely to be.