For t2/m3/c3/c4/r3/i2/d2 instances:
m1 small, medium, and large instances tend to perform higher than expected. c1.medium is another freak, at 800 MBit/s.
I gathered this by combing dozens of sources doing benchmarks (primarily using iPerf & TCP connections). Credit to CloudHarmony & flux7 in particular for many of the benchmarks (note that those two links go to google searches showing the numerous individual benchmarks).
The large instance size has the most variation reported:
Burstable (T2) instances appear to exhibit burstable networking performance too:
The CloudHarmony iperf benchmarks show initial transfers start at 1 GBit/s and then gradually drop to the sustained levels above after a few minutes. PDF links to reports below:
t2.small (PDF)
Note that these are within the same region - if you're transferring across regions, real performance may be much slower. Even for the larger instances, I'm seeing numbers of a few hundred MBit/s.