Shii tzus tend to be very dominant, especially the males.. but the females are "controllable"
Small dogs are simply very easily intimidated, and they try to counter it with agression, to scare the "opponent" away.. and showing that they aren't prey..
Maltezers are probably the best option.. dog races all have their own "specifications".. my dog is an Airedale Terriër and is known for it's stubberness.. and intelligence (though she never uses that part of her brain)..
Chihuahuas are simple bred as accesoiries for rich women..
As small dogs, that chihuahua, pinscher, yorkshireterrier are risky.. (though if you raise them the right way everything will be just fine.. for 99% ofcourse, like with everything else)
foxterriers and jack russels are also very dominant most of the time..
Best choices are dogs like the shii tzu (though a bit risky) the maltezer etc.. and if your thinking a tiny bit bigger, you can get the miniature version of my dog (airedale terrier..) that's the welsh terrier.. Those are really great, caring dogs.. very intelligent for a "small" dog.. they are jack russel sized, but not so fat.. quite slim..
I'm not telling you what to buy, but just keep it in mind

I do a lot with dogs, and learned quite a lot from them..
If your not planning to have pups, you could sterrilize the dog.. it makes it less agressive.. but it might make it lazy.. and they tend to get fat (lack of hormones..)
And one other thing.. sounds stupid..
But it's really annoying when there is a mini-dog around the house.. You always have the scary feeling that your going to step on it. (had one for 2 weeks to baby sit it) and they tend to walk between your feet.. like they're suicidal or something.
Sure, someone may one day kill me with my own gun.
But they'll have to beat me to death with it because it's empty.
A computer once beat me at chess, but it was no match for me at kick boxing.