Since I keep seeing bad reviews that make little or no sense, I add my 5 stars here.
I've been using Transmission since version 0.7 and it has always been the best one, in my opinion.

It's stable, with no frills, free, easy to set up and control while in use.
I also tried many time to compare its speed with other torrent clients: there were no big differences with the same files under the same conditions.

Those who claim it crashes or it doesn't work well should chek their settings better, because here it works just fine.