Possibly fido, as they have the most reasonable price plans overall. But to be honest, I have never used a pay as you go service because I find I use my cell phone more often than I plan to, even when I make a pact to myself.
Though you should just check individual carrier sites and look out for the best price plans.
pay as you go is a pain in the wazzu. Something like the Talk 20 with telus will be better for you if you don't use the phone much. I went that way from pay as you go because the buggers kept stealing my minutes. I would buy a card not use it all and poof my money was gone.