Kinda depends what you're after really, its kinda like gloves or a helmet, everyone prefers their own thing.
I have an Evoc Freeride Trail 20 which serves me well but is £107 so fairly pricey. I also have a Camelbak Octane 20 lightweight bag for mtbing. I've found between 15 and 25 litres is about what most people use. If its just for trials things like a bladder arn't really neccesary as you're not riding with the bag on. If you plan on doing any mtbing it might be worth a look. Alot of climbing/walking bags are well made (Osprey etc) but for round town it can look like you've got lost from the moors.
Chainreaction has a fair selection to look at and see what you're after.