I've driven all of these.
Fiat 500 is my vote by a wide margin—the Sport, not the Turbo or Abarth. The Sport has the stiffer suspension setup vs. the Pop or Lounge models. It's shorter than the Mini by quite a bit, and has great visibility. You can confidently slice traffic and park swiftly in it, and it's at least as fun as the Mini. Solid on the freeway, too. Car & Driver's long-term test in their latest issue had no unscheduled stops, which bodes well for reliability (and ease of service is why I'd skip the turbo. that packaging is super tight). I love the looks, and the interior too. Skip the nav system, it's clunky and weird.
It'll be great fun at autocross—might be a good contender in the new Street class. And it's the second-cheapest in your lineup.
The only fun thing about an iQ is that, if you overcook it with understeer and mat the pedal, the traction control kicks in, cuts power and gives you about a half-second of four-wheel drift before the stability control kicks in. Otherwise, the transmission is garbage, it's expensive, the packaging is terrible, and there's no way you could even fit two small teenagers in the back. To fold down the rear seats, you have to remove the headrests. And there's no good place to put them, so you just kinda throw 'em on the floor. It also gets crappy highway gas mileage. For the money, anything is a better car. Literally anything.
I think the Smart wins on purchase price and supreme parkability. Staggered wheel fitment! Hell, you could lease one for $99/month. But that's about all it does well.
Mini loses on price, but wins at the autocross. Depends on your term of ownership, you might not have to worry about the cost of repairs, and resale value is high. But if you're keeping it a long time, repairs will be pricey. Loses on parkability in this matchup.
I absolutely do not recommend a Miata for a city runabout. I've done it. Squeezing through traffic is good, but visibility with the top up is crap and it's longer than but has a shorter wheelbase than the Mini, which makes it harder to park. And it's low, which means other drivers—SUVs, primarily—can't see you and like to change lanes quickly. It's also disconcerting being eyeball-level with the lug nuts on delivery trucks.
B-segment cars like the Fiesta, Yaris, Versa, et. al. are practically Suburbans against these. Huge.
But hey, have you thought about the Chevy Spark? (It's not electric.) Skip the vinyl "leatherette" upholstery. It's as cheap as the Smart, but pretty fun to toss around, and has four doors. Super practical. A competent driver. A little weird looking, but I think it works well in green and blue. Seriously, add it to your list.