Azureus or Transmission are your choices... both have ups and downs, try them and decide for yourself what works best for you.
I used them both and find myself switching between this 2 periodically.
Yes there are more like Tomato etc. but I haven't used those so I can't tell
If you're a masochist like me and like using a program that has practically no support whatsoever, you can download Bits on Wheels, the project is practically dead, but the client has a pretty 3D representation of your file downloading, kinda like the swarm tab in Azureus, only better.
I've tried the others but I come back to Azureus despite it needing Java. It has options the others can't touch and I love that I can do selective downloads from a torrent. Makes life, oh so much, easier when only needing a file or two from a bundle.