Early days torrent scene ran on proprietary adware clients like BitTorrent itself, µTorrent, and BitComet but nowadays open source clients are more popular and recommended by most trackers. I'm on a pretty big private tracker and almost everyone (+95%) is using qBittorrent, Transmission, or rTorrent. Though Vuze was also one of the earliest clients and it's also open source for some reasons it never really became pop…
I actually use all three you mentioned in different contexts.