Private trackers you won’t have to worry about fake torrents / malware / letters from your ISP telling you stealing is bad.

For starters, many have mentioned freeleech torrents to build ratio, you can also add stuff you already have by downloading the same release from their site and setting the directory to where you already have the files (this only works for releases that are identical though), if your internet is fast enough, you can snag just released torrents (the more popular you expect it to be the better) and reseed to other people as they get it. This only works for torrents that are within a few minutes old though.

Past that look into getting a seedbox or set up an old/cheap computer to run constantly and download your torrents to that, and just transfer them to your main pc as you need them.