It depends. I think the conventional wisdom is with a single action style semi-auto or revolver, the middle of the pad of the trigger finger is placed on the trigger (and this includes double action only, safe action, like Glocks, M&P's and XD's among others). Double action revolvers and Double action/single action semi-autos should employ the crease of the first joint on the trigger. Jeff Cooper wrote about this several times. I think he however had pretty good sized hands.
However, and this is a big however, there are exceptions, disagreements and so on about this subject. It appears most firearms are designed around these principles. A good repeatable grip is first and paramount. One big factor is the size of the hand and also the length of the fingers (one can have a big hand but stubby fingers). Personally I follow the traditional guidelines but have struggled over this problem for years and have made a number of changes in guns, grips and shooting styles to accomodate it. I use the pad of the finger on many DA/SA semi-autos because my fingers aren't long enough to achieve a proper grip and reach the crease. Although it worked, it wasn't ideal and I no longer have any DA/SA autos. I use Spegel boot grips on my N-frame and use the pad on my trigger finger and it works very well; but I shoot all other double action revolvers at the finger crease.
Clint Smith has larger hands and long fingers; he easily can reach the crease for N-frames (a very long trigger reach with standard grips) but also uses the pad for 1911's, etc. Elmer Keith and John Taffin used(s) the pad of their trigger fingers due to their hand size. Mas Ayoob seems to prefer the crease for everything but I understand may have backed down on that to some degree lately.
Grip size and configuration and trigger reach are some of the most important considerations in handgun use. To be frank, it is rarely discussed or understood. I think a lot of people buy handguns that are unsuitable for them for these very reasons and have unsatisfactory shooting experiences as a result.