qBittorrent - One of the most popular ones. Completely free to use and ad-free. I use this!
Deluge - Also free but more simple (less features) than qBittorrent. Deluge often comes default with Linux distros since it's very lightweight and easy to use. I've only used this on Linux because it came pre-installed.
Avoid these:
uTorrent - Ads and sketchy activity. Most people have ignored it for the past 5+ years. At one point, it tried to mine bitcoin off of user clients. They may have changed, but most people have already switched away.
Vuze - Ads and paid membership option. Again, not worth because you're better off spending that money on a Seedbox.