Another vote for an enamel paint, they take a while to dry in comparison to something like a lacquer paint, but make up for it in durability. Honestly, ive had good results with just the rustoleum rattlecans, wouldnt paint a car with it but it works well enough on furniture projects and the like ive never once found myself thinking "man, i should look for something that works better".
Whatever you do, DO NOT forget the primer. I dont care if its clean metal or if the can claims its a 2-in-1 paint and primer, prime it anyways. If the chairs are taken back to bare metal, Rustoleum again sells a self-etching primer meant for use on bare metal that i quite like, it actually eats into the metal a tiny bit when you spray it, does its own surface prep almost. If theres still paint on them, go for the universal bonding primer instead, just make sure to prime the surface before painting.
Honestly, more important than the type of paint is the prep of whatever your painting. Id recommend sanding or stripping back to bare metal, just to get the most optimal surface to paint. After that, thoroughly degrease everything, if these are used chairs you dont know whats on them that could interfere with a finish. If the weathers nice, take em outside, hit them with some dish soap and water and hose em down. Before painting, give everything a wipe with a rag and some acetone, and wear gloves so you dont transfer any oils from your hands to the surface. Once everythings cleaned, hit it with the primer and remember that when it comes to spraying paint, less is more. Thin, light coats are the ticket. After priming, go for painting, and after you paint them be sure to give whatever you use adequate time to cure. Enamel paints take a while, and can be somewhat fragile until they finish curing. Something like an epoxy paint would likely cure faster, but are supposed to be harder to work with, so be sure to weigh which ones more important.
Again, dont skip the prep-work in favor of looking for the best paint, just go with a quality name brand and youll get good results if you start with a good surface