I only use this punch.
Aluminium Cartomizer Punch V4
It is more expensive than others but it has a cobalt tip. I have been giving mine heavy use (like sometimes punching up to 15 cartos a week) for over a year and my tip is still as sharp as it was the day I got it with no distortion of the carto.
+1 on this punch..totally worth the money with the carbon tip and big window so you can see exactly where you're punching. This is all I use as well.

