People like most of the default apps in macOS mainly because they are free and simple. However, their simplicity often prevents these apps from having the necessary features that third-party alternatives can provide. Here are some of the best alternatives to default Mac apps.

Besides its minimalist design and ease of use, there are several reasons to use Safari on your Mac instead of other browsers, including tight integration with other Apple devices, fast performance, easy-to-use memory and power efficiency. However, simple is not always good, depending on each person's workflow, so some people have replaced Safari with the Arc browser.
Arc is a free third-party Mac browser designed by The Browser Company and built on Chromium. Due to its novel approach to web browsing, it will take you some time to get used to it, but after that, you will find this new browser to be a productivity miracle.
2. Raycast (instead of Spotlight Search)

If you were a previous Windows user, you probably fell in love with Spotlight when you switched to Mac, because it saves you many clicks every day. Shortcuts can be used Cmd + Space to quickly search for files, launch applications and even perform calculations. Overall, Spotlight is great, but there are better alternatives.
Raycast is one of the essential applications that should be installed before using a new Mac. This third-party system-wide search engine provides most of Spotlight's basic features. However, Raycast is better than Spotlight in some aspects.
3. Warp (instead of Terminal)

The Terminal utility allows you to perform a variety of tasks on your Mac via the command line. However, if you are not familiar with using Terminal, you may find it difficult and prone to making mistakes, potentially harming your system. Learning how to use Terminal on your Mac is a good way to start. Alternatively, you can use Warp as a more user-friendly alternative.
Warp is an intuitive Mac terminal alternative with a number of useful features that make working with the command line easier. One of my favorite features in Warp is the auto-suggest/auto-completion feature, which suggests commands and folders as you type. Its autocorrect feature also makes it easier to detect errors.
4. Fantastical (instead of Apple Calendar)

If you're a heavy calendar user, chances are you've considered using a third-party calendar app at some point. That's because Apple's Calendar app can sometimes feel limited. Fantastical is a well-designed and feature-rich Apple Calendar alternative that's worth considering.
Similar to the native Calendar app in macOS Sequoia, Fantastical integrates seamlessly with Apple's Reminders app, making it easy to keep track of tasks and events in one place.
5. SoundSource (replaces default volume control)

As a former Windows PC user, the lack of a built-in volume mixer on macOS was one of the biggest complaints among users. By default, you can set different volume levels for each app through its settings or audio controls. However, besides being inconvenient, some applications also rely on the system volume and do not have their own settings.
That's where SoundSource comes in. SoundSource is a third-party application that lets you control your Mac's audio output. A favorite SoundSource feature is app-based volume control, which conveniently allows volume levels for different apps to be adjusted in one place. For example, you can use this feature to quickly lower or mute the volume of Apple Music during a video call.
6. Path Finder (instead of Finder)

Path Finder is a feature-packed file management application that offers more control and customization options than Finder. Overall, this application helps track file paths and organize files much more effectively. In addition, the application also provides useful features such as Drop Stack which allows you to temporarily hold selected files so you can easily drag and drop them where you need them.
Size Browser is another convenient feature that helps you easily sort files by size, delete large files, and free up disk space. Last but not least, Dual Pane view allows you to work with two folders side by side, simplifying tasks like copying or moving files between folders.
7. IINA (instead of QuickTime)

IINA is an open source and intuitive media player that offers a simple and modern interface with support for multiple formats. While support for almost any type of video and audio file is reason enough to switch from QuickTime, IINA still offers many other useful features. For example, it gives you full control over video playback speed, brightness, contrast, saturation, and audio equalizer settings.
Furthermore, it can stream videos from popular streaming platforms like YouTube and supports picture-in-picture mode, allowing you to continue watching videos while doing other tasks.