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.