They are different and it depends on your personal preference which one is best. I prefer Platypus (by quite some margin) and they are only sold in Overlander (12th floor Hollywood Plaza Mongkok, google for their other stores).
Some brands are more prone to leaking than others (the ones that come fitted in Osprey packs have a particularly bad reputation for that).
The hydrapak ones have weird bite valves - I don't like them, but you might.
Another leading brand is Camelbak - they are the blue ones - not sure who sells.
Basically it's like anything - if you buy the cheapest you see it'll probably break, and if you were relying for the water on a long hike, having it all leak out is annoying (or dangerous, depending on where you are at the time).
There are many outdoor stores that stock decent brands - Overlander, RC Outfitters, Chamonix, Protrek are the main ones. If you buy from one of those, it should be decent kit. If you get it from some smaller stores, be sure it's a well known brand.