The Nelson Longflap isn't perfect but it's what I use.
You can get a fair whack in there and then stuff the same amount under the flap. I keep a dry bag in mine and if I'm picking up some food shopping on the way home it goes in there and gets bungee-ed on top. I think it looks rugged.
Still, image-wise the bag has baggage. Looking a it one way, t's a bit 'traditional' like something Norman Tebbit would approve. On the other hand, whenever I'm following a cyclist with a Nelson I feel reassured, relaxed and relatively confident of a level of competence.
It's not really designed for modern things like laptops but it carries them fine and with a bagman fitted you get this springy,bouncy support which is probably good for hard drives, microchips and that kind of stuff.
It keeps stuff dry.
I've used messenger bags, panniers and rucksacks and they have their place but not for me so often.
I don't know whether anyone's mentioned porteur racks yet but there's been a big trend for them in London at least for a couple of years or more. Every other bike in Peckham has them now.