Well, I've only had a few tanks, but so far my favorite is the 75g. It's a nice footprint for getting some depth without being too deep to work with and can hold quite a variety of small/medium fish or a couple larger fish depending on the sizes. It's a nice stepping stone from the smaller, limited tanks and the massive monster tanks. Most people can find a place to fit a 75g too but might not for a 125g.
I also like my 20g long. It's a small tank, but the length gives it more stocking ability than regular or tall 20g tanks. Plus, it looked really nice with a forest of najas grass and anubias.
My other tanks were/are a 10g for Mom, 2.5g bowfront, 1-1.5g, 44g corner, and a 16g tall (custom?). The only one of these I really hate was the 1-1.5g one because there was so little that I could do with it. I'm not particularly sold on the 2.5g because it's also limited, but it's planted only for now with plans to add either a nerite snail or a couple amano shrimp.