Probably an inflammatory response, but I stand behind it:
IG Stormtroopers!
I know they get a lot of hate (or perhaps apathy is more accurate), and that the classic argument goes "Vets are better!" to be rebuffed with "
ST's fill a different role!" but at the end of the day, it's one of the most flexibly deployable units in the game; and you don't have to decide how you're going to deploy them beforehand.
They excel at popping up where and how you need them to and performing surgical strikes against the enemy, which is fantastic in a codex that is otherwise full of big blasts and (generally) foot-slogging or transport-riding grunts. I never leave home without 'em!