I have considered this qustion many times and have yet to have a real good list. As has been mentioned before, the unit you have a tag for is the best one... That said, how many of the units have been personally scouted for big bulls? I personally was raised in the shadow of the Pahvant and spent many years hunting deer on the unit before it had the big bull reputation. In the past 10 years I have spent maybe 10 days total on the unit and have seen some bruiser bulls. As for the other top units, I have seen big bulls on Manti and spent 3 LE hunts with friends on Dutton and killed 3 bulls that scored north of 330. All really nice bulls.
I have not been to San Juan, Cache North, Monroe, etc... So I am not qualified to opine... I agree with KTC, any unit south have the center of Utah has the potential to grow the monsters.
All who wander are not lost.