Let me chime in:
1) 1 Gbps (theoretical)
From what it looks like here, 1 Gbps Ethernet wins! Sadly, it doesn't; by looking at the overhead of packets and buffers, etc. You can find out that it'll get up high, but not that high. So you easily say over 800 Mbps.
2) 480 Mbps (theoretical)
USB 2.0 has gone a long way from when it started. Sadly, from what I have read in the USB 2.0 Specification Documents, there is really no set common clock speed for transmitting data. So you'll see a range of 12 Mbps to 480 Mbps.
3) 800 Mbps (theoretical)
FireWire (IEEE 1394) is by far a favorite among Mac users. It is fast, and it's specification documentation does say that it can get up to 800 Mbps. Sadly, like 1 Gbps Ethernet, it can't quite get there due to physical limitations. So, it's good to say over 650 Mbps.
Lastly, the one thing that a lot of Mac users forget is that Time Machine also works on what kind of computer you're running. So think about your machine and specific hardware specs it has. If you are running an Intel Core 2 Duo, a high L2 cache, max'd out in RAM, and not many programs running on your machine, you'll see a higher throughput.
So saying which is best/fastest is a bit of a catch 22. It all depends!
I find an external drive using FireWire with a machine max'd out and no applications (or just a couple) running will have a good speed.
I recommend trying each and gather data from them and check yourself if one is better than the other.
