qBittorrent

The default recommendation for most people, because it gives you a clean, ad-free, uTorrent-style client without the bundled junk the original picked up. It’s open-source and cross-platform with a built-in search and a web UI when you need it. The caveat isn’t really the app, it’s that a torrent client alone does nothing for privacy; pair it with a VPN or your real IP is exposed to every peer. Start here unless you have a specific reason for a lighter or more specialized client.