That depends on how much magic there is in the world. If half the criminals have spells or supernatural abilities, the regular cops had better be ready to deal with them. If magic is pretty rare then it can be dealt with by "SMAT" teams.
I agree with Len here. It all dependso n the amount of magic and how hidden it is
For a generic setting with spirits, casters, weres, vampires, fae and demons in the open but not super common ala the Anita Blake books I would suggest
.357 revolver (probably a S&W 686+ with 7 seven rounds for luck) with speed loaders -- types of ammo might include steel AP (for Fae or guys in bullet proof vests ) explosive (lead styphenate) or incindiary for lots of stuff, silver (for weres) and of course lead -- Why not an automatic A revolver is more reliable with wierd ammo than an automatic. I am not sure silver bullets would work in a glock at all in fact
Shotgun with lots of wierd payload ammo -- probably a Remington 870P with 8 round tube, and side saddle shell carrier 9and one on the stock too) with two of each round -- optionally a Winchester Defender might be used
Rifle with wierd loads in trunk -- pick 1 -- I suspect a setting with big monsters would upgun to a .308 instead of the standard assault rifle in use now -- a Remington 700BDL or Winchester 70 -- most likely a package deal and fromn the same company the shotgun is from
Charms of various worth -- if mages are not monster only in the setting the guns may be blessed as well with charms for reliability and "not getting stolen"
Salt
"sovereign remedies" -- a little kit of stuff that works in the setting -- I would suggest -- Powdered silver and Wolfsbane (for Weres) Garlic and Concencrated Wafers (for Vamps) Blessed Salt (for Demons) Powederd Iron, Rowan and Red Thread (for Fae) Holy Water 2 vials -- if Holy Water is easy to make and effective -- lots of holy water
Clejuso Super Heavy Restraints -- not on the belt though -- these are to heavy to lug easily
Setting Usefull Holy Symbol