I use Deluge on my seedbox and uTorrent 2.2.1 on my PC, simply because it doesn't choke when loaded with loads of torrents.
I don't really know what I'd recommend for Windows, to be honest, other than uTorrent 2.2.1. Everything else I've tried just hasn't really been able to handle a large load effectively, with the exception of Tixati, but that one just has a rather bloated interface from the get-go.