bittorrent clients

Recommended:

qBittorrent [cross-platform] > https://www.qbittorrent.org/ [recommended for Windows users!!]
Deluge [cross-platform] > https://www.deluge-torrent.org/
rTorrent (+ ruTorrent) [linux only] > https://rakshasa.github.io/rtorrent/ ; ruTorrent > https://github.com/Novik/ruTorrent

Do not use µTorrent!:
https://www.engadget.com/2015/03/06/utorrent-bitcoin-miner/ – Popular torrent client can steal your CPU cycles to mine bitcoins, Engadget

while this miner issue has been closed, µTorrent still displays ads. You can turn them off in the settings as far as I know, but since there are better clients available I’d suggest you use one of them.

Some useful links:

Comparison of BitTorrent clients – Wikipedia, the free encyclopedia
The Best Free BitTorrent Clients – How-To Geek