I recently bought this Doughnut backpack from Wardrow. It's available in loads of different colours and the 14" one is just about big enough for my smallish laptop (it's ever so slightly bigger than a sheet of A4 paper), plus things like lunch, water bottle, phone, purse, etc.
I've had a few things from Wardrow and they have a wide selection, good prices and are reliable. However, they're in mainland Europe so I don't know how easy it is if you need to return anything.
I looked at Kanken and Roka and, while I know they're hardwearing, I didn't like how 'basic' Kanken are for the money. I couldn't find anywhere where I could order a Roka with free returns and I wasn't sure about the size etc and I didn't want to buy something unless I was quite sure about it. Although as it happens, I've found them for sale in a National Trust shop since then.
I've been in the market for 'the' backpack for sometime but wasn't sure what I wanted, but finally settled on something a bit more casual, so not 'naice handbag but a rucksack' or a proper hiking one, something a bit in the middle, and a bit 'fun'. I mainly use it for work, but it's not very corporate, many people are a bit unconventional/eccentric so it's not inappropriate.
The other one I looked at was the KNOMO Beauchamp (available to see in John Lewis) but that was when I decided I didn't want something more formal/smart.