Not found an "ideal" day pack yet so i alternate between 2.
For hikes under 2 hours i tend to take my Maxpedition Jumbo (0412B) with Janus extension pouch (8001B).
What i like most about this pack is that by swinging it forwards i can access all the pockets without stopping or removing the bag.
I also like the fact that it's got many pockets and organisation compartments, so i can quickly put my hand on what i want.
Unfortunately it's really uncomfortable, i am ok for about 2 hours after that it really starts becoming a literal pain in the neck.
I've found that by really tightening the belt strap it's more comfortable as the hips then take more weight, but still it's not comfortable for longer hikes.
It's also very heavy.
For longer hikes i use my Osprey Manta 25
By far the most comfortable day pack i've ever used.
With it's stand off type design you don't even get much of the wet back syndrome either.
It's also top quality and fairly light weight.
I don't use it on shorter hikes as it's a ball ache getting to stuff.
Apart from the hip belt pockets it's virtually impossible to retrieve anything from the pack without removing it.
Even then it's a nightmare to find anything as there are no loops or elastic to hold anything in place, so you tend to just get a soup of everything in your bag resting at the bottom.
It does have pockets so i am able to store things like the FAK in certain pockets but it's nowhere near as convenient as the Maxped bag to get to anything.
So it's really a compromise between comfort and accessibility to the bags contents for me.
I added 2 shoulder strap pouches to the Osprey so i can access my phone, and GPS, but even the hip belt pockets are difficult to access as they are right back near your bum.
As i', putting on and taking off layers,putting the dog on and off her lead, giving her water etc i probably have to access my pack more than most here, but still i'd buy a slight heavier Osprey pack if it had loop and compartments to stow stuff.