I have used all of the above addons (except the last one Seerah mentioned), and if you want simple, but still organized, I'd go with Tbag. I'm not that big on all-in-one bags, personally, but I LOVE organization. I like the "virtual bags" that you can create with Baggins. It's the same concept as an all-in-one bag that gives you access to your entire inventory with the push of a single button, but it also comes with a minimap button (and/or a broker/fubar/titan plugin) that allows you to open up specified bags that you create as well.
I have my bags split into "Trophies", "TradeGoods", "Consumables", "Quest", "Gear", and "Other". Within those bags, I have it further broken down to organize the things I put into those bags (i.e.-my "TradeGoods" bag has sections for "Metal", "Leather", "Herbs", "Cloth", etc....). This lets me choose to open only the parts of my bag I need to deal with. If I'm out on a quest and have an item I need to use that isn't showing up as clickable in my quest tracker, I can open my "Quest" bag and leave it open while I'm doing that particular quest instead of having ONE GIGANTIC BAG taking up my entire screen so I can have access to one little item.
This ability, more than anything else, is what keeps me coming back to Baggins all the time. It's the same concept as ArkInventory and most other bags that let you split your inventory into sections/categories, but it gives you the option of and all-in-one bag, or as many "virtual" bags as you want/need.
The setup takes a little playing with and knowhow to learn (not as simple as Tbag, but IMO easier than ArkInventory), but once I got the hang of it, there was no looking back.
Here's a quick picture of what I'm talking about...the bags and bank of one of my latest alts:
