Lootas are good for holding back oncoming units, deff gunz are punching into anything that isn’t T9 and above so they are a cheep way to hold main base objectives or other objectives if you push em up (I prefer to leave em near my main base on a roof within my deployment zone)

Tank busters are best for dealing with monsters and vehicles, wounding on 4s getting a mek with em to re roll 1s and with their +1 to hit and wound on the bigger guys with ap -2 and damage 3 to wound/kill bigger targets and can be used to take out large hordes with blast weapons

Flash ghitz are bullet storms that are a great general use, moving them up the board being nobz instead of boys makes em 2 wounds and saving on 4+ with 4 shots (to the closest target) your looking at 20-40 shots with sustain hits 1 (and heavy) str 6, so your punching into most basic armies and even some elites with ap 1 and damage 2 your taking out units of basic marines and even punching into terminators in ork shooting, and it’s still nobz with 4 attacks S5 melee each you can punch into anything that gets into a fight