Maxpedition Fatboy for me. Tactical black, of course.

Before that it was the good old GI Buttpack with a canvas shoulder strap.
Used that for years, the Fatboy for a little over a year now.
I usually carry a 1 litre bottle of seltzer to work, it sticks out the top a little, no biggie. Also have trashbag, thin disposable poncho, note book, paracord, dustmask, headlamp, gerber honesteel (prybar), shrade buzzsaw trapper, 2 bandanas and some other crap in it.
Organization is much better with the FB.
I like lumbar bags, but they don't work well for me in the winter over a heavy coat. Always feel like they are sliding down.
Better in the summer than a daypack though, no big sweaty spot on your back. Smaller sweaty spot along your belt line
The thing missing from the FB that is on the GI bag are the straps on the bottom of the bag. You could keep a (real) rain poncho there, easily accessible, wet or dry. Also worked well for sweaters, etc.
Without filling your main compartment.
You could also hang a canteen or ammo pouch on the sides of the GI bag, but that gets heavy quick.
Good luck!