Well, in response to the question of what is the MUST have unit for any army, I decided to respond as the Ork general in me (I figured a million other SM players would be giving their opinion - fyi by must have marine unit is the assault marine with power fist sergeant).
In the ever present Mech build that is all to common in todays 5th edition scene, the Orks have their biggest defect exposed. Their lack of "reliable" anti (light) armor units become a problem. Sure, tank bustas are OK, but I never see them being used, and never use them myself. The flaw is they need to get up nice and friendly to be most effective, and thats not all that good with a 6+ save. So, enter the Lootas (in cover for a 4+ save). With a unit of 8, you have the potential to pump out 24 S7 shots. Even if only 8 hit, your taking out a transport a turn. Assume we get 16 shots off - 6 hit lets say - then 2 should penetrate and 1 glances. You should have yourself a blowed up rhino. I take a sqaud of 10 in most every game I play, just for what I described. Taking out transports IS the most vital tactic any ork general must abide by. Exposing the nice warm units inside the transport makes them ripe for a 2nd turn WAAAGH assault, which is of course the bread and butter of da boyz.
Also, I have a nice Horus Rising AND False Gods lack library book im awaiting to give away. Just need to get that 25th follower for Horus Rising! Trust me, its worth putting up with my post to get a free copy :-) (again, giving away my horus heresy series novels to my trust followers - need 25 to give away Horus Rising (which was an awesome read)).