Tough call. The one I'd knock out is the Kahr. It's alright, but nothing special. In my humble collection, I have among others a P13.45 (similar to the Springfield, I guess), a G23 (quite frankly not sure which the 36 is. I think it's a .357 or a single stack, but lost count at G27), and a 229. Don't have a USP yet (although it's on my list of things to buy; I just haven't decided which one I want yet).
If I remember correctly, I bought the Glock 23 first, then the Para-Ord, then the 229.
Sounds like it's just a matter of time before you have all 5, so I'd get the gun with the highest capacity first, along with some hi-cap mags. Chances are you'll be able to buy a Kahr or a Springfield with full (<10) capacity mags in the near future, but hi-caps will be hard to find.