All depends what you want from it. My own view is the best bag is the one that offers the features you need at a price you are prepared to pay and fits you. Do you want side pockets, do you want/need a bladder section? Do you want/need access to the main compartment from the bottom and the top of the bag? What sort of "external" pockets do you want/need? Do you want/need all the sewn on straps that some bags have? Do you want the ventilation "system" some bags have?
When you have some idea of the features you want/need - pop into a decent shop and try some on. I reckon you will find a bag that suits from (in no particular order) Berghaus, Osprey, Gregory, OMM, Lowe Alpine or Deuter. In truth, all those manufacturers (and others) produce good bags but they produce different designs from each other and within their own ranges. Having recently purchased 25 - 30l bags for myself and my daughter, we found the Osprey bags over complicated with too many straps and pockets that we would not use (and weren't prepared to pay for). Others take a different view. I have a Lowe Alpine 35l bag which I find too heavy. My daughter ended up with a Gregory Jade which seems a well made simple enough bag which fits her, is easily loaded (unlike a Karrimor we have and which is a pain cos the openeing is too small and the bag is bell shaped) and I got a Deuter (which has access from the bottom amd the top of the main compartment and a zippable means of dividing the main compartment - I can carry waterproofs in the lower part or my biggish camera). Both have bladder sections and both can carry poles.
Try to visit a shop with a good supply. Do not discount second hand. Birders upgrade their binoculars and their are some very good bargains to be had. Bins are definately something you should try before you buy. The differences between one set and another can be very subtle and disproportionate to the price differential.
I have bought used bins from Focus Optics and In-Focus. Good advice and service from both. There are, of course, other outlets.
I use a Garmin Legend C with TOPO mapping. The GPS is an older model than yours and so far as I know is fairly similar save that yours has the micro-card facility and an altimeter and a more sensitive aerial. Anyway, the TOPO mapping is not what I recognise as OS. It certainly does not compare to a 50,000 let alone a 25,000 map. What it does have is contours and a bit of detail but really it is not worth £150. In my view the GPS unit is fine but the TOPO mapping is not essential and given the choice again, I would save the money or put it towards "proper" OS mapping.
If what you want is contours, the SMCoffer a free and entirely legal download from