I must have gone through 30 or so different tank bags over the years, I have about 10 that I keep around. 3\4 of which I do not use since the bikes they were bought for have been sold.
What kind of bike is it for? That would determine the shape.
What do you use it for? Permanent fixture on the bike or a day bag that will mount to the bike?
Do you NEED a map pocket?
I like Held and Marsee personally for brands, but I also have the new AXIO, as well as the GL Fandango and the Kriega tank adapter (but I use it with an Osprey crampon pocket). Again, it depends on the bike it's going on.