First, don't buy any bag, sling, backpack, or holster b/c any of us recommend it. For this kind of stuff, fit is personal. A backpack that rides well on me may give you back spasms. A sling that works for Max may be unusable for you. Once you've got an idea of what options are on your list, go to a camera store or a Best Buy and try some on. See how they fit. See how easy you can use/access them.
Second, as someone else posted above, list the equipment you want to carry.
Third, think through how you're going to use it. Realistically a backpack is great for getting gear from Point A to Point B. It's not so great for doing shooting between A & B. It's also not great if you're in a car (say...a safari or maybe driving around like Yellowstone). If you want something to hold lens while you shoot, then you probably want a sling. But most slings aren't so good for laptops. Messenger bags can be good for a laptop, a body and maybe 2 lens plus a few other items. What this means is that there are times when you want a backpack (b/c you're hauling gear for 2 hours and then you're going to shoot) and then there are other people who are going hiking and will shoot as they hike (so they need to be able to access their gear without taking a backpack off). Will you need it to be water proof (b/c you'll be hiking outdoors in weather)? Attach a tripod? Carry clothes and food with it as well.
For me, I have a messenger back that I added an insert into that will hold a body, 3 lens, and a laptop. I have a Tamrac holster (which is what I use most of the time). I have a Case Logic backpack. And when I fly, my shooting gear (including soft box, stand, tripod, speed lights, backdrop, lens, bodies) get divided between a backpack (which also holds 2 laptops) and a soft-sided duffle (which is carryon sized). It works for me. I'm sure it wouldn't work for many others. But provide more details about what you have and how you're going to use it...then people can offer better advice.