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More about a lack of knobbies and something reasonably puncture proof and durable. I've been using Schwalbe Marathon Supremes for the past few years and am happy with the compromise between rolling resistance and puncture protection (they even used to make a matching 20" tyre for the front). Obviously Marathon Pluses are great if you never want to fix a puncture, but they ride as if you were pushing through treacle.
Those WTB Horizons do look like an option though, thanks!
I have seen one with a WTB horizon which seems to fit fine (also with mudguard). As long as it fits any tyre is cargo bike suitable, no? Couriers here use Schwalbe Shredda and the original Kendas on the front, both of which being pretty soft after all.