Ferrocium rod! Tried 'em, used 'em, trust 'em. LMF Scout and Exotac Nanostriker
Here's why I didn't choose the others:
Mag Block.
I used to have a Doan Bar type, and though I had no problem with the flint, I had great issues about getting mag shaving in a controlled fashion. They would ping off in all directions resulting in the block getting smaller, and my face getting redder - but no nearer getting a tinder supply built. I have heard the trick of using duct tape to catch the shavings - but didn't have huge success with that either. Magnesium does burn very hot - but very quick, so you still need to have other tinders anyway for the magnesium to light. That said I do still carry a mini-match too (mini-version)
Fire Piston.
Good for getting a coal with the right tinder - but only if you can manage to maintain a seal on the piston. A stray piece of grit or other damage can render your pistion useless. A fun item, I have no doubt (though never tried one) but you should always have a back up for this. Novelty use only in my opinion
Waterproof Matches
I've got some in a survival tin (along with a flint/striker as back up). Matches are great because they give you a flame rather than just a spark or coal - but they only last so long. Fine as part of a system, but not a recommended stand alone solution in my opinion. Save them for the tough to start tinders that keep shrugging off the spark from your striker
Butane Lighter
As a smoker this is my (several times) daily "go to" ignition device. I cannot ever remember trying to get a smoke using a flint & steel, steel wool on a 9V battery or friction firelighting (I have used solar/lens though). Great little items - cheap last a good time and are usually reliable. I've had plenty of occasions though where gas has run out, valve has failed, flint has pinged out, piezo has failed, I've dropped it and it's gone Phuttt (cracked head off) etc etc etc. Again fine as part of a system
The only one of these that I would be happy carrying on their own would be ferro rod. It works, lasts for ages, works in wet conditions (providing tinder is OK) you could even try signalling with the sparks at night. I'd never stop anyone using any other methods, but would always recommend they had a ferro rod too.
YMMV