There are several good options. A number of factors can help narrow the field:
How much are you willing to spend
How small do you want to go? Is it a case of the smaller the better or are you willing to take more size for a gun you can shoot and run better?
Do you have a preference for a certain design, i.e. SA/DA, single action only, striker fired, etc.
Just of the top of my head there following are guns I would consider:
Kahr: K9, CW9, P9, CM9, PM9, MK9 etc (I really like the PM9/CM9 if small is a top priority)
Sig: P938, P239
S&W: Shield
Springfield: EMP, XDs,
Walther: PPS (I really like this one)
Ruger: LC9
I'm sure there are more I'm not thinking of
All of the above are viable options. Some are more proven than others. It is more a matter of personal tastes and which attributes one favors than one being per se better than another.
For a primary, belt carried gun, I really like the PPS and the EMP. The Walther is an underrated gun IMHO.