I love talking about lands.
From a set with a few other reasonable competitors, Mobilized District is a clear winner for best Standard land in War of the Spark.
Lands that become creatures have a long history of being good. I would like to remind people that the following card saw considerable Constructed play both times it was printed.
I don't know what's worse: the activation cost, the Mirrodin goofballart, or the Tempest interpretive dance art. It really doesn't take a lot to make a creature-land a solid card, and Mobilized District is solidly across that line.
Where on the creature-land scale does Mobilized District fall?
While comparing a colorless land to a multicolor land isn't entirely fair, just being a 3/3 for four mana would put Mobilized District into a tier of reasonable but not amazing options. Vigilance is certainly a far cry from hexproof, so the creature is a step down from Lumbering Falls. I would put it around par with the worst couple of Oath of the Gatewatch creature-lands, namely Needle Spires.
But I'm also not being entirely fair to Mobilized District. Four mana is the worst-case expense.