There are so many fabrics, styles, and cuts to choose from when selecting a garment for your printing needs. In the majority of cases, you can take a shot in the dark and still deliver a successful product.
However, the more you know, the better.
Below is a photo I took of a tank I've owned for almost 3 years. The screen print was done on a Bella baby rib knit racerback tank, with a stretchy contouring structure (i.e. tight-fitting). It's super soft and the shirt itself has held up well wash after wash. As you can see over time the ink has faded, but that is to be expected (shown as is on the left).
On the right is what the design looks like when I wear it. Due to the nature of rib knits and curve-hugging fabric, prints on this variety of shirt will stretch and often times crack and peel away. I should add that this is especially noticeable when worn by individuals with a little...extra going on, like myself.
Left: laid flat; unworn. Right: photo taken while worn.
Though the result screen printing on a baby rib knit is not ideal, it is still considerably O.K. when you account for the age of the print. We also advise against using a 2x1 knit for printing, as this result shown above will be much worse.
If you do choose to go with a rib knit, make sure your printer knows their stuff! An experienced printer will know exactly what to recommend--and sometimes it might mean suggesting a digital print instead of a screen print. Always shop around to get the best advice!
So, keep this tidbit of information in mind when selecting a garment to print on, especially if you plan on retailing your design. Best of luck!