qBittorrent is available as free, ad-less software licensed under GPL with source code available on GitHub. Being open source allows anyone to check if the software has no malicious code, is secure and respects privacy.

qBittorrent covers basics such as Prioritization, Selective Downloading, DHT, PEX, LPD and Magnet URIs as well as Broadcatching, Sequential downloading and Search. It includes MSE/PE for security. It has graphical, web and command line interfaces. Has UPnP, NAT-PMP, NAT traversal for automatic router configuration. Has UDP tracker and µTP data transfer protocol support. Even covers Tracker Exchange, IPv6, web seeding, Embedded tracker, Super-seeding and proxy servers.

Allows to download the first file of a series prior to the following items.

Obtain lists of files in Magnet before prompting the user for a download location.

Torrents can be automatically read from a folder.

qBittorrent supports sequential downloading that allows users to download files in sequential order. That means that media such as video or audio can be streamed a the same way it can be with any other streaming service.

Can configure to what ratio (amount uploaded/amount uploaded) the torrent should be seeded. You can also configure on whether you want them to be paused or removed after this.

May also set this directly in the ~/.config/qBittorrent/qBittorrent.conf:

[Preferences]
Bittorrent\MaxRatio=2.5
Bittorrent\MaxRatioAction=0  # 0=Pause, 1=Remove

Algorithm that helps downloaders to be able to become uploaders more quickly.

Uses python plugins made and maintained by users that can be found on the official websites.

qBittorrent can be used to create a torrent to share large files/folders with other people.

The remote API and web interface works well. There is headless version too (called qbittorrent-nox).

Add RSS feeds for automatic downloading. Apply filters to only download what you want.

Can be run on a headless Linux (for text-only mode from the command-line without requiring X11). The package is commonly named qbittorrent-nox (and it's officially supported by qBittorrent).

What i love most about qbit is the "SEARCH" option thru major torrent-websites.

IP filtering blocks set IP addresses. It's usually used to block communications with certain companies that are against p2p activity, etc.

Makes it possible for BitTorrent peers to learn about new trackers for a swarm they have joined. Ideally ending up with every peer knowing about every tracker used for the torrent

qBittorrent now allows tagging individual torrents. So, besides Categorising torrents (ISO, Video, Podcasts, etc) you can tag torrents too (Jul, Aug, Sep or whatever).