Which country has the lowest flight time requirement for the issuance of a pilot certificate?
Here in the US you can get your sport pilot in 20 hours but that is a pretty heavily limited license, your basic VFR PPL will require at least 40 hours here in the US. You should note that this is flight time which is generally tracked on the hobbs meter (positive oil pressure or master switch on depending on configuration). There is a lot of other time in ground school, flight planing, pre-flight checks, and waiting for weather to pass that you will undoubtedly run into. 

Looks like Canada requires 45 hours and thats day only. The UK also requires 45 hours.

These hours are for VFR only. An instrument rating, which is not a separate license but an additional rating on the pilot certificate, will take more hours. Some countries limit night flying privileges without an instrument rating, and if you intend on doing any serious flying its a good idea to have.  

As mentioned by Ron in the comments, not all licenses are valid everywhere. Some are recognized with limitations, and some can be transferred depending on what you are trying to do. Some privileges may be recognized if you are flying to an area to visit etc. Other situations may require more training or filing different types of flight plans. Almost everyone is on ICAO format these days.