Well, the ACU builds the factory that spams out Engis, one of which builds more factories, one of which goes to T2 to build T2 Engis and so on.
Early on your ACU should be heading out of the base to cap forward mexes and reclaim from T1 battles, otherwise it's a waste of it's strength. Only once enemy T2 starts showing up in depth do you then pull them back and start upgrading them.
It's been a while, but I think in terms of "Build Power per Amount of Mass to Aquire" the T1 Engi is the flat out best unit, followed by T2 Engi, T3 Engi, ACU Pod Upgrade, ACU T2, ACU T3, Upgraded SACU, Plain SACU.
The fully upgraded ACU has the most build power of all (tied with Upgrade SACU), but for the amount of mass you're paying you'd technically get more power out of the same cost in T1 Engis (with diminishing returns due to space and pathfinding).
But generally, I don't upgrade the ACU until I've already got T3 Factories. Even then sometimes I just don't bother because I'm behind in SMD and they'll be 30 feet underwater for the rest of the game.