what firing mode is best suited for a heckler and koch UMP 45?

Answer 1

well... that depends on the situation of course.

Basically, ammo is a valuable commodity in any circumstance, so the basic setting should be single fire.

especially with the UMP 45 it is always worth to take serious aim, because the weapon shoots more or less straight over several hundred feet, which far from customary with this type of firearm.
of course, sometimes the only option is to plaster the evironment with steel, in hopes to either hit an undetected target or at least force the opposition into cover. the relatively small clip capacity of the UMP 45 in combination with the absence of a short burst setting makes this gun a less than optimal choice in these moments.

Answer 2

short, controlled bursts.... always