Missing info... depends what and how much equipment you are carrying. How about budget too.
For a medium, I like my lowe pro slingshot 200AW. Even big enough for my pentax 645 system.
For a lightweight bag, I have a plain jane messenger bag purchased from the good will store that I use for all sorts of trips. When used as a camera bag, I insert a domke 3 sectional padded insert. ** This is the one I use the most ** Doesn't look a camera bag and thus is generally "safer' from potential theft.
For more junk, I use a lowepro 400AW shoulder bag with two shoulder straps to even out the weight across both shoulders. There are two larger sizes made by lowepro but I couldn't imagine carrying any more stuff on my shoulder.
I also have a backpack Tamrac Expedition 7. Carrys a boat load of stuff but I haven't used it in years, which means I should
ebay the darn thing. It is a well built backpack but I find backpacks in general so inconvenient.
For airplane travel, there is only one in my opinion... the Pelican 1510 with padded divider set.
You don't have to spend the money to get a good bag ( as in my messenger bag with inserts).