Depends on the intended usage, for defensive EDC in modern urban environment, a Mini Uzi with a can. small, light, folded up and hanging from a shoulder strap it will hide under a well cut sport coat. I have not tried a HK PDW set up, but would rank it similar.
urban assault, police use, liability concerns, (that is, civilian injuries in a lawyer filled environment) the MP5 family. the trigger group and sights make it perhaps the most accurate Subgun ever.
third world conflict/insurgency etc full size UZI or sten. nothingelse comeclose in terms of simplicity and cost effectiveness. UZI's cost to the Israeli Gov't was something like $37 per. When it was a frontline arm, the savings over other options allowed Isreal to defend itself and not go broke. During WWII, Stens Mk 2 were made for about $9-11 each. and later marks cost slightly more as the design was refined. although some would disagree, with later magazine improvements, stens became quite reliable.