RC-5 may be most common in some places. Note however that is a Philips protocol. If your area has mostly Philips equipment then RC-5 is likely to be the most common. However it is necessary to consider that there are other types of protocol including the SONY type, the NEC type, (which over all in the world probably eclipse the number of implemented Philips units more than 2 to 1) plus there are many other types.

Take some time and search about IR on this forum. It has been dicussed here MANY times. Also there google search engine will turn up more information than you can digest in many days.

Michael Karas