Most Departments Issue Service Weapons
Therefore I can only assume you are looking for a personal piece that will closely replecate what you end up working with. I think Glock still has somethingn like 60% of the national LE market so being that you most likely don't know which agency you will be working for I would reccomend a G22 in a full size gun that will closely replecate what you are most likely to be working with.
I own a G27 subcompact and a G23 compact. (not being in LE I get to use guns that I like and fit my hands and needs). The full size is not a good grip for me therefore I do not own a full size glock. I will tell you that I don't think you can buy a better pistol for the $.
Congrats on your profession, Thank you for your upcoming service, and welcome to TFL!