There is no BEST torrent client, but I recently found a great alternative to Utorrent when Utorrent doesn't work.
I generally use Utorrent for everything, however, Utorrent has a few annoying quirks, and sometimes can't download about 1 out of 1000 torrents.
When that happens, I suggest using Qbittorrent. It is quite a bit like Utorrent, but does things in a slightly better way. For instance, with Qbittorrent, you don't have to stop a file before re-checking it. It does that automatically.. And when it's done checking you don't have to manually restart it, it restarts automatically.
There is one problem thing about Qbittorrent that bothers me though. The file downloads and uploads are considerably slower than with Utorrent.
Utorrent is even faster than Bittorrent which is made by the same people and looks 100% identical. Without doing a lot of testing, I would rate the speed of Utorrent a 10, Bittorrent a 7 and Qbittorrent a 4.
Maybe there are some settings in Qbittorrent to speed it up.. or maybe the makers of Utorrent will take a look at Qbittorrent and implement some of the improvements they have made.
I've tried them all - and continue to try new products as they hit the net. For the past few years the rock star of clients is BiglyBT. It's free, it has anything/everything you can think of, and it has ZERO free-ware/ad-ware/spy-ware contained inside. I highly recommend you give it a try. It manages my massive torrent collection on my Windows PC like a boss. I'm seeding 1,932 torrents at the moment, 9 are active - and it hardly puts a dent in my PC's ram. You can find it at https://www.biglybt.com/