What’s the least important 400-series/municipal closed access highway that’s still open in Ontario? I’d say Highway 2A.
I'd have to say Burlington St/Nikola Tesla Blvd in Hamilton. It may have been useful in the 1960s when the steel mills were booming, but I don't really see the utility of it now. It's the weirdest quasi-freeway that I've ever seen, with the dips and rises that it has.