Generally speaking any of the big brands make a decent shampoo, so long as you do a thorough job it's the steps following especially claying that are going to make the biggest impact so you have to look at cost factoring in dilution ratio especially if you are doing this for coin.
Citrus Wash & Gloss is fairly cheap in Canada ($32.20) per 128oz with good dilution ratio of between 1:256 and 1:512 depending on how dirty the vehicle is and if you are looking to strip LSP or not. Mr. Pink I've not used but is $32.99 per 128oz and claims a better dilution ratio in some places (1:640) and 1:256 in others. In either case these shampoos are highly recommended and reviewed. The prices here are higher and options fewer. Duragloss 902 is $34.50 per 128oz, and it can't beat the dilution ratios of the CG stuff from what I've seen. Most shampoos are in that price range though Griot's and Poorboys are $45 or so (all prices using eshine). Heck even the 64oz Megs gold class at Canadian tire is $19.99 regularly or $14.99 on sale. You re better off going the CG, Duragloss or Optimum route in Canada than the consumer level Megs.
Find one of the bigger names at a relatively good price and go with it. Your personal preferences will differ than other people. Personally I've stuck to CG for the shampoo but the main brands sell out on eshine often but it looks like oldetown has the bigger names at slightly lower prices and autoobsessed is a little more expensive. The catch is oldetown doesn't seem to have the CG options.
What is best? Any well reviewed brand (CG, Optimum, Duragloss) on sale.