I've used rust bullet, por 15, zero rust, chassis saver, and Rust Tough.
I have to say the easiest to use was chassis saver, by magnet paints. Also very tough.
All except Rust Tough are MCU's (moisture cured urethane) All were tough. Toughest was Rust Bullet, applied with two Alum undercoats, and one satin black top coat, but the finish was not so great. I brushed it on my frame and firewall and I've always gotten bubbles from the Alum undercoat. Rust Bullet address the issue somewhat in their FAQ. You might have better results with spraying or using the foam brushes.
All were thoroughly degreased and I used "metal prep" or other Phos agent.
I base my opinion of "toughness" on how difficult it was to remove (wire brush) on some parts I over-painted.
It really doesn't matter, I found, which one you use. Frankly, if I were doing it again, I'd just use a bunch of this:
Krylon: Products:
Cheap. Works great. Easy touch up (rattle cans at Autozone) ...