According to those statistical whizzes at Fight Metric:

One of the best ways to measure effective defense and dominance is to look at the number of strikes a fighter absorbs. Because of variable fight lengths, the easiest way to consistently calculate this is to figure the number of strikes absorbed divided by the number of minutes fought. That gives you the number of strikes absorbed per minute of fighting, which we abbreviate as SApM. Here are some SApM figures for the five current UFC champions:

Anderson Silva: 0.71 SApM
Georges St. Pierre: 1.01 SApM
BJ Penn: 1.23 SApM
Rashad Evans: 1.39 SApM
Frank Mir: 2.7 SApM

Let's wrap things up with two more fighters' numbers:

Fedor Emelianenko: 0.59 SApM
Lyoto Machida: 0.64 SApM

Both Fedor and Machida have proven harder to hit than all of the UFC's champions. And it's not like there is a shortage of data on any of these guys. These numbers are from their entire careers. Does it mean Fedor and Machida are better than the other fighters? Not necessarily...this is just one subset of their entire fighting profile. But it is interesting to note that Fedor and Machida come in #1 and #2 in terms of the hardest guys to hit in MMA history.

There's no arguing with statistics.

UPDATE: As known wise-ass Luke Thomas points out in the comments, Rami backs off from making any broad categorical claims about the meaning of this statistic. But I don't. Note that Fedor is pretty much the consensus #1 MMA fighter of all time and that the order of the UFC champions according to  their SApM stats is eerily consistent with their standings in the eyes of  fight pundits. Everyone knows that Anderson Silva is the best UFC champ, GSP is next and B.J. is third. Now we know why. It's their SApM score. From now on, don't even try to argue with me unless you've got some SApM scores handy to back up your claims.