In no particular order, I think these are United's most important hubs:
ORD (Midwest)
DEN (Mountain West)
IAH (South)
EWR (East)
SFO (West)
IAD is also important but I was trying to pick one from each region, and if only one, EWR over IAD.
That would leave LAX and GUM. I can write a long story for each, and I know United considers importance for each, but when push comes to shove, if in a hypothetical, these two would be the front runners for de-hubbing.