Qbittorrent, Transmission etc. The Transmission daemon can be installed headless with negligible system load on a vast number of devices, from Raspberry Pi-like and smaller SBCs to Linux/BSD NASes, then operated from remote through the web interface or a phone app.
Then you probably don't want a free service that costs money to run where they can only make money by converting most users to paid or monetizing your information in a country where you are unlikely to have an attorney whilst operating what amounts to a honeypot for every government on earth.
For desktop use from within Plasma/KDE I'm happy with Ktorrent. Feels very intuitive, and has no problem saturating a 1GB/s pipe, and doesn't slow the system down, while doing so.
(At least not mine, which are old and almost obsolete but have enough RAM)
Otherwise follow the links from there to qBitTorrent, or its mentions from other commenters here. Am not fond of transmission at all. Feels slow and sluggish in comparison.