Ah, the joys of being punched and kicked as a hobby. You might think if one is training in the dojo 3 or 4 times a week and doing conditioning at least twice a week, that it might get a bit easier. Unfortunately, the body you are born with, and its pain receptors, never really changes a great deal. The pain you feel today is the pain you'll feel tomorrow. What you learn in a martial art, however, is how to compartmentalize that pain and ignore it.
For me, the worst strike is the low leg kick (gedan mawashi geri.)
I can take a pretty good amount of body shots, and if I'm focused, I'm pretty good at not getting stunned from hits to my front. But a good low leg kick is a nightmare for me. And as a result, I work on it every time I train. I have been building muscle in my quads and hamstrings as much as possible, because come May I need to be able to handle those hits to my legs.
There are three strategies to a low leg kick:
1. Block. This basically involves putting your shin in the way of their shin. This is, not surprisingly, also painful. But it's not as debilitating as having your quad muscle cramp up from a strong blow.
2. Dodge. This means getting your leg out of the way of his kick. This is obviously less painful, but generally is harder to do and involves more stamina. And if you are unsuccessful, you end up getting hit anyway, and the opponent now knows a bit about how to use your dodge timing against you.
3. Channel the energy. This is the hardest one, and involves a lot of understanding of both your body, the opponent's kick, and how energy flows. This one requires that at the moment the kick hits, you channel that energy down and into the floor. It's not about "chi" or any movie bullshit: it's really just about how you move to ensure that the blow is nullified. This is the hardest, but it can also be used to make an opponent waste an otherwise useful blow and let you set up the next one.
In case you were wondering how a low kick might be debilitating, here's a video of one of the champions of the low kick, Hajime Kazumi.